Lithium ion polymer secondary battery its electrode and method for synthesizing polymer compound in binder used in adhesion layer thereof

ABSTRACT

The present invention provides a lithium ion polymer battery comprising an electrode having excellent adhesion and electrical conductivity between a current collector and an active material layer, cycle capacity maintaining characteristics being improved, wherein a bonding layer, which bonds the current collector to the active material layer, is stable to an organic solvent in an electrolytic solution and is excellent in storage stability, and also, it is made possible to suppress the corrosion of the current collector by a strong acid such as hydrofluoric acid generated in the battery.

TECHNICAL FIELD

The present invention relates to a lithium ion polymer secondary batterywhich has a bonding layer between a current collector of an electrodeand an active material, and a method for synthesizing a binder used inthe bonding layer of the battery.

BACKGROUND ART

In recent years, thin batteries have been in increasing demand with thespread of portable devices, such as video cameras and laptop computers.A typical thin battery is a lithium ion polymer secondary battery whichis formed by laminating a positive electrode and a negative electrode.The positive electrode is made by forming a positive electrode activematerial layer on the surface of a sheet-like positive electrode currentcollector, and the negative electrode is made by forming a negativeelectrode active material layer on the surface of a sheet-like negativeelectrode current collector. An electrolyte layer is interposed betweenthe positive electrode active material layer and the negative electrodeactive material layer. In this battery, a positive electrode terminaland a negative electrode terminal are provided on the positive electrodecurrent collector and the negative electrode current collector,respectively, for leading out a current generated by a potentialdifference between the two active materials, and the laminate formedthereby is hermetically sealed in a package so as to form the lithiumion polymer secondary battery. The lithium ion polymer secondary batteryuses the positive electrode terminal and the negative electrode terminalwhich lead out of the package as the battery terminals to providepredetermined electrical output.

The lithium ion polymer secondary battery having such a structure hashigh battery voltage and high energy density, and it is viewed as verypromising. Bonding layers are often interposed between the currentcollectors and the active material layers. Characteristics which thebonding layer is required to have include sufficient force of adhesionwith the current collector material, sufficient force of bonding withthe binder contained in the active material layer, stability in thepresence of an organic solvent in the electrolytic solution, excellentlong-term storage stability, thermal stability to remain without peelingwhen exposed to high temperature and electrochemical stability to endurerepetitive charge-discharge cycles, but there has not been a solutionthat meets these requirements.

For example, it is necessary to increase the surface areas of thepositive electrode sheet and the negative electrode sheet in order tofurther increase the discharge capacity of the lithium ion polymersecondary battery. However, simply increasing the surface areas of thepositive electrode sheet and the negative electrode sheet has a drawbackof making the electrodes difficult to handle because of the largesurface areas. Solutions may be conceived for this problem, such asfolding or winding the large positive electrode sheet and negativeelectrode sheet to a predetermined size. When the positive electrodesheet or the negative electrode sheet are folded or wound in a laminatedstate, the positive electrode sheet or the negative electrode sheet isbent along the folding line, thus causing the positive electrode sheetor the negative electrode sheet to peel off from the electrolyte layer,such as polymer electrolyte layer, which leads to a decrease in theeffective surface area of the interface between the electrode and theelectrolyte, resulting in decreasing charge capacity and internalresistance of the battery, thus deteriorating the cycle characteristicsof the charge capacity. Also, there has been a problem in that thecurrent collector peels off the active material layer formed on thepositive electrode sheet and the negative electrode sheet due to bendingalong the folding line. Moreover, there has been a problem in that thepositive and negative electrode active material layers expand andcontract as the positive and negative electrode active material layersstore and release lithium ions in the charge and discharge cycles,resulting in a stress that causes the active material layer to peel offthe current collector. To solve the problems described above, a methodhas been proposed such as a bonding layer being formed between theactive material layer and the current collector so as to prevent thelayers from peeling off and adhesion from being reduced by means of thebonding layer.

The bonding layers, that are interposed between the positive electrodeactive material layer and the positive electrode current collector andbetween the negative electrode active material layer and the negativeelectrode current collector, are required to have both the function ofholding the two members together and the function of providingelectrical conductivity, and are therefore formed by dispersing anconductive substance in a polymer material used as a binder that holdsthe two members together.

Prior art techniques that aim to solve the problems described aboveinclude one in which a bonding layer is interposed between the activematerial layer and the current collector so as to prevent the layersfrom peeling off and adhesion from being reduced by means of the bondinglayer, as disclosed in the prior art documents (1) to (5) as follows:(1) Japanese Examined Patent Publication No. 7-70328 discloses a currentcollector coated with a conductive film mentions of a binder and aconductive filler. This invention names phenol resin, melamine resin,urea resin, vinyl resin, alkyd resin, synthetic rubber and the like asthe binder material. (2) Japanese Unexamined Patent Publication No.9-35707 discloses a constitution in which a negative electrode materiallayer containing a binder made of powdered carbon and polyvinylidenefluoride (hereinafter referred to as PVdF) is formed on the negativeelectrode current collector, and a binder layer made of an acryliccopolymer containing a conductive substance mixed therein is formed onthe negative electrode current collector. This invention achieves a highbonding effect by using the acrylic copolymer having high strength ofbonding with copper for the negative electrode plate whereon thenegative electrode current collector is formed from a copper foil. (3)Japanese Unexamined Patent Publication No. 10-149810 discloses aconstitution in which an undercoat layer is formed by applying apolyurethane resin or an epoxy resin between the active material layerand the current collector. This invention improves adhesion between theactive material layer and the current collector in the electrode byforming the undercoat layer from polyurethane resin or epoxy resin,thereby improving the cycle capacity maintaining characteristics of thebattery.

(4) Japanese Unexamined Patent Publication No. 10-144298 discloses aconstitution as a bonding layer made of graphite and a binder betweenthe negative electrode current collector and the negative electrodeactive material layer. According to this invention, graphite containedin the bonding layer improves the efficiency of the negative electrodein accumulating charge. (5) Japanese Unexamined Patent Publication No.9-213370 discloses a constitution in which graft-polymerized PVdF isused as a binder for the electrolyte portion and the electrolyte layerof the battery active material. This invention improves the efficiencyof making contact with the current collector by using graft-polymerizedPVdF as a binder for the electrolyte portion and the electrolyte layerof the battery active material.

Characteristics which the bonding layer is required to have includesufficient force of adhesion with the current collector material,sufficient force of bonding with the binder contained in the activematerial layer, stability in the presence of an organic solvent in theelectrolytic solution, excellent long-term storage stability, thermalstability to remain without peeling when exposed to high temperature andelectrochemical stability to endure repeated charge-discharge cycles.

However, the technique (1) has a problem in that butyl rubber, phenolresin or the like used as the binder is corroded with the electrolyticsolution and peels off. In the technique (2), although adhesive strengthbetween the negative electrode current collector and the negativeelectrode material layer can be increased by forming the bonding layercontaining, as a major component, acrylic copolymer containing aconductive substance mixed therein between the negative electrodecurrent collector and the negative electrode material layer because anacrylic copolymer has high strength of bonding with PVdF contained inthe negative electrode material layer and the negative electrode currentcollector, there has been a problem in that the acrylic copolymer iscorroded by the electrolytic solution and the negative electrode currentcollector peels off the negative electrode material. Although thetechnique (3) is claimed to increase the peel-off resistance and thenumber of 80% capacity cycles in the case in which polyurethane resin isused as the undercoat layer, the effect is not practically sufficient.When an epoxy resin is used, the epoxy resin is corroded by theelectrolytic solution, and therefore there is a possibility that theactive material layer will peel off the current collector.

In the technique (4), although satisfactory adhesive strength betweenthe bonding layer and the active material is achieved since the bondinglayer includes a material similar to the binder contained in the activelayer, the strength of bonding with the current collector is notsatisfactory, and it is comparable to that in a case in which the activematerial layer is formed directly on the current collector. Also,because the electrolytic solution infiltrates the binder, there is aproblem of weak bonding strength between the bonding layer and thecurrent collector. In the technique (5), the active material layer canbe formed directly on the current collector without using a bondinglayer since the graft-polymerized material that has high strength ofbonding with the current collector is used as the binder of the activematerial layer, but it has a drawback in that solvents that can be usedare limited since the polymer is difficult to dissolve. Also, because itis difficult to completely remove the solvent from the inside of thebattery, there is a possibility that the solvent remaining in thebattery will cause an adverse effect on the battery performance.

In the second prior art technique to solve the problems described above,powdered carbon is dispersed as a conductive material in the bondinglayer. However, the powdered carbon does not provide sufficientelectrical conductivity, and it is necessary to increase the weightratio of the powdered carbon to the binder, (powdered carbon/binder), inorder to obtain satisfactory electrical conductivity. When theproportion of the powdered carbon in the bonding layer is increased, theproportion of the binder in the bonding layer decreases and the contactarea of the binder with the current collector and the active materiallayer decreases due to the large volume of the powdered carbon, thusresulting in insufficient adhesive strength.

As the third prior art technique to solve the problems described above,a battery electrode is disclosed in which a bonding layer is formed in apattern of dots, stripes, or a grid between the current collector andthe active material layer (Japanese Unexamined Patent Publication No.11-73947). The battery electrode is provided with a paint to form thebonding layer applied thereto by spraying or printing. The area in whichthe paint of the bonding layer is applied is in a range from 30 to 80%of the active material holding area of the current collector.

In the battery electrode constituted as described above, since thebonding layer is formed in a predetermined pattern between the currentcollector and the active material layer, adhesion between the twomembers can be improved without impeding the exchange of electronsbetween the current collector and the active material layer, therebyimproving the cycle characteristics. Specifically, adhesion between thecurrent collector and the active material layer is maintained by thebonding layer that has the predetermined coating pattern, so that theexchange of electrons between the current collector and the activematerial layer is carried out smoothly in portions that are not coated,thereby keeping the electrical resistance low.

As another technique that belongs to the third prior art techniquedescribed above, a battery electrode is disclosed in which a binder thatconstitutes the electrode is uniformly dispersed in the electrodematerial (Japanese Unexamined Patent Publication No. 7-6752). Thiselectrode is produced by forming an electrode containing a bindermaterial dispersed therein on a current collector and drying theelectrode, followed by pressure forming and further heat treatment.

A high performance secondary battery having excellent charge capacitycharacteristics, particularly cycle characteristics, can be produced byusing the electrode that has the constitution specified in the twodocuments as described above.

However, among the third prior art techniques described above, thebattery electrode of the prior art described in Japanese UnexaminedPatent Publication No. 11-73947 requires it to form the bonding layer ina pattern of dots, stripes, or a grid, and there is a problem in that itis very difficult to form the bonding layer. Also, there remains aproblem in that, in the case in which the area of portions which are notcoated, where electrons are exchanged between the current collector andthe active material layer, is relatively large, sufficient adhesioncannot be maintained in the portions and peel-off occurs.

In the electrode of the prior art disclosed in Japanese UnexaminedPatent Publication No. 7-6752, the polymer used as the binder or thepolymer electrolyte that provides binding effect is completely dissolvedin the solvent, and this is mixed uniformly with other materials such ascarbon and the active material, thereby preparing a coating slurry. As aresult, sufficient adhesive strength cannot be achieved between thecurrent collector and the active material layer, thus the problem ofdecreasing charge/discharge cycle characteristics of the battery remainsto be solved. This problem is supposed to be due to the presence of alarge quantity of powdered material, such as carbon, that is added tothe coating slurry, in the interface between the current collector andthe active material layer.

DISCLOSURE OF THE INVENTION

A first object of the present invention is to provide a lithium ionpolymer secondary battery which is excellent in adhesion and electricalconductivity between the positive electrode current collector and thepositive electrode active material layer and between the negativeelectrode current collector and the negative electrode active materiallayer, and also has improved cycle capacity maintaining characteristics.

A second object of the present invention is to provide a lithium ionpolymer secondary battery in which a bonding layer interposed betweenthe positive electrode current collector and the positive electrodeactive material layer or between the negative electrode currentcollector and the negative electrode active material layer is stable inthe presence of an organic solvent in an electrolytic solution, and hasexcellent long-term storage stability.

A third object of the present invention is to provide a lithium ionpolymer secondary battery capable of suppressing the corrosion of thecurrent collector with a strong acid such as hydrofluoric acid generatedin the battery.

A fourth object of the present invention is to provide a method forsynthesizing a binder used in a bonding layer of a lithium ion polymersecondary battery which is excellent in adhesion and electricalconductivity due to the bonding layer interposed between the positiveelectrode current collector and the positive electrode active materiallayer or between the negative electrode current collector and thenegative electrode active material layer.

A fifth object of the present invention is to provide an electrode for asecondary battery which is excellent in adhesion and electricalconductivity between the current collector and the active material layerand is capable of improving the cycle capacity maintainingcharacteristics of the secondary battery, and a secondary battery thatuses the electrode.

A first aspect of the present invention is a lithium ion polymersecondary battery comprising a positive electrode comprising a positiveelectrode current collector, and a positive electrode active materiallayer, which contains a first binder containing a polymer compound and apositive electrode active material, provided on the surface of thepositive electrode current collector; a negative electrode comprising anegative electrode current collector, and a negative electrode activematerial layer which contains a second binder containing a polymer thatis the same as or different from that of the first binder, and anegative electrode active material, provided on the surface of thenegative electrode current collector; and an electrolyte,

wherein a first bonding layer is interposed between the positiveelectrode current collector and the positive electrode active materiallayer, and a second bonding layer is interposed between the negativeelectrode current collector and the negative electrode active materiallayer, the first and second bonding layers contain both a third binderand a conductive material, while the third binder contains a polymercompound obtained by modifying either or both of the polymer compoundscontained in the first and second binders or a polymer compound havingany of repeating units of the polymer compounds, with modifyingmaterial.

According to the first aspect of the present invention, since thepolymer compound contained in the third binder, which is contained inthe first and second bonding layer, is a polymer compound obtained bymodifying either or both of the polymer compounds contained in the firstand second binders which are contained in the positive electrode activematerial layer or the negative electrode active material layer, or apolymer compound having any of repeating units of the polymer compoundswith modifying material, the bonding layers have high force of adhesionwith the positive electrode active material layer or the negativeelectrode active material layer. Adhesion of the bonding layers to thepositive electrode current collector and the negative electrode currentcollector is also remarkably improved because the third binder containsa polymer compound obtained by modifying either or both of the polymercompounds contained in the first and second binders or a polymercompound having any of repeating units of the polymer compounds withmodifying material.

A second aspect of the present invention is a lithium ion polymersecondary battery comprising a positive electrode formed by providing apositive electrode active material layer containing a first binder and apositive electrode active material on the surface of the positiveelectrode current collector, and a negative electrode formed byproviding a negative electrode active material layer containing a secondbinder which is the same as or different from the first binder and anegative electrode active material layer on the surface of the negativeelectrode current collector,

wherein a first bonding layer is interposed between the positiveelectrode current collector and the positive electrode active materiallayer, a second bonding layer is interposed between the negativeelectrode current collector and the negative electrode active materiallayer,

the first and second bonding layers contain both the third binder andthe conductive material, and

the third binder contains a polymer compound obtained by modifying afluorine-containing polymer with a modifying material.

According to the second aspect, since the third binder contains apolymer compound obtained by modifying the fluorine-containing polymercompound with a modifying material, adhesion of the bonding layers tothe positive electrode current collector or the negative electrodecurrent collector is remarkably improved compared with the binder of theprior art.

In the first aspect, either or both of the polymers contained in thefirst and second binders is preferably a fluorine-containing polymercompound. This fluorine-containing polymer compound, or thefluorine-containing polymer used in the third binder in accordance withthe second or third aspect of the invention is more preferably afluorine-containing polymer compound selected frompolytetrafluoroethylene, polychlorotrifluoroethylene, PVdF,polyvinylidene fluoride-hexafluoropropylene copolymer and polyvinylfluoride.

The fluorine-containing polymer compound is preferablypolytetrafluoroethylene or PVdF because of high durability to theelectrolytic solution.

In the first or second aspect, the modifying material is preferablyselected from ethylene, styrene, butadiene, vinyl chloride, vinylacetate, acrylic acid, methyl acrylate, methyl vinyl ketone, acrylamide,acrylonitrile, vinylidene chloride, methacrylic acid, methylmethacrylate, and isoprene, because good adhesion with the currentcollector can be obtained.

More preferably, the modifying material is acrylic acid, methylacrylate, methacrylic acid or methyl methacrylate.

In the first or second aspect, the thickness of the first and secondbonding layers is preferably from 0.5 to 30 μm.

When the thickness of the first and second bonding layers is less than0.5 μm, the ability to protect the current collector decreases,resulting in lower cycle characteristics of discharge capacity. It alsomakes it difficult to disperse the conductive powder uniformly whenforming the first and second bonding layers, resulting in an increase inthe impedance. When the thickness of the first and second bonding layeris more than 30 μm, since the volume and weight of the portion whichdoes not contribute to the battery reaction increases, energy densityper unit volume and unit weight decreases. The thickness of the bondinglayers is preferably from 1 to 15 μm.

In the first or second aspect, it is preferable to contain 0.1 to 20% byweight of a dispersant in the first and second bonding layers.

By adding 0.1 to 20% by weight of the dispersant in the first and secondbonding layers, it is made possible to uniformly disperse the electricalconductive material in the first and second bonding layers. Examples ofthe dispersant include acidic polymer dispersant, basic polymerdispersant and neutral polymer dispersant. When the content of thedispersant is less than 0.1% by weight, there is no difference in thedispersion of the electrical conductive material from that in a casewithout the dispersant, and the effect of adding the agent cannot beobtained. Adding more than 20% by weight of the dispersant does not makea difference in the dispersion of the electrical conductive material anddoes not contribute to the battery reaction, and there is no need to addan excessive amount of the dispersant. The content of the dispersant ismore preferably from 2 to 15% by weight.

In the first or second aspect, it is preferable that particle size ofthe conductive material be from 0.5 to 30 μm, a carbon material having agraphitization degree of 50% or more is used as the conductive material,and a weight ratio of the third binder to the conductive materialcontained in the first and second bonding layers, (thirdbinder/conductive material), is from 13/87 to 50/50.

When the weight ratio is less than 13/87, the proportion of the thirdbinder in the bonding layer is too low to obtain sufficient adhesivestrength. When the weight ratio is higher than 50/50, the proportion ofthe conductive material in the bonding layer is too low to provide forsufficient electron mobility between the current collector and theactive material layer, resulting in increased internal impedance. Theweight ratio of the third binder to the conductive material is morepreferably from 14/86 to 33/67.

A third aspect of the present invention is a method for synthesizing thethird binder contained in the bonding layer of the lithium ion polymersecondary battery, wherein the third binder is synthesized by modifying(a) either or both of the polymer compounds contained in the first andsecond binders in the first aspect, or (b) the polymer compound havingany of repeating units of the polymer compounds, or (c) thefluorine-containing polymer compound in the second embodiment, with amodifying material, and the proportion of the modifying materialcontained in the third binder is from 2 to 50% by weight based on 100%by weight of the third binder.

By controlling the proportion of the modifying material added to thethird binder in the range described above, it is made possible to obtainthe third binder which is excellent in adhesion and electricalconductivity. When the proportion of the modifying material in thebinder is less than 2% by weight, the strength of bonding with thecurrent collector becomes weak and, when the proportion is higher than50% by weight, it becomes difficult to dissolve in the solvent. Theproportion of the modifying material in the binder is preferably from 10to 30% by weight.

Modification with the modifying material is preferably carried out byirradiating the polymer compound (a), (b), or (c) with radiation andmixing the modifying material with the irradiated polymer compound,thereby causing graft polymerization. Modification with the modifyingmaterial may also be carried out by mixing the modifying material withthe polymer compound (a), (b), or (c) and irradiating the mixture withradiation, thereby causing graft polymerization.

Irradiation of the polymer compound (a), (b) or (c) with radiation ispreferably carried out by using γ-rays so that an absorption dose of thepolymer compound (a), (b) or (c) is from 1 to 120 kGy.

The absorption dose less than 1 kGy or higher than 120 kGy leads toproblems of decreased adhesive strength of the binder.

A fourth aspect of the present invention is a lithium ion polymersecondary battery according to the first or second aspect, wherein

the first and second conductive materials contain a metal or partiallyoxidized metal having a particle size of 0.1 to 20 μm, and a weightratio of the third binder to the first conductive material contained inthe first bonding layer, (third binder/first conductive material), and aweight ratio of the third binder to the conductive material contained inthe second bonding layer, (third binder/second conductive material), arefrom 13/87 to 75/25.

Since the first and second conductive materials contained in the firstand second bonding layers contain a metal or partially oxidized metal,good electrical conductivity can be obtained with smaller quantity addedto the bonding layer than the carbon material used as the electricalconductive material in the prior art, since the metal has goodelectrical conductivity. Furthermore, by controlling the particle sizesof the first and second conductive materials, the weight ratio of thethird binder to the first conductive material contained in the firstbonding layer, (third binder/first conductive material), and the eightratio of the third binder to the second conductive material contained inthe second bonding layer, (third binder/second conductive material), inpredetermined ranges, satisfactory adhesion, high electricalconductivity and cycle capacity maintaining characteristics areobtained.

The first and second conductive materials preferably contain mixtures oralloys of one or more kinds selected from the group consisting ofaluminum, steel, iron, nickel, cobalt, silver, gold, platinum,palladium, and partially oxidized material of these metals.

It is more preferable that the first and second bonding layers containacidic polymer dispersant, basic polymer dispersant or neutral polymerdispersant.

A fifth aspect of the present invention is an electrode for a secondarybattery, comprising a current collector and an active material layerformed on one or both surfaces of the current collector via a bondinglayer containing a polymer binder, wherein a portion of the polymerbinder exists in the bonding layer in the form of particles and avolume-mean particle size of the particulate polymer binder is from 1 to100 μm.

The particulate polymer binder that exists in the bonding layer existstogether with the conductive material, that exists in the form ofparticles, in the interface between the current collector and thebonding layer and in the interface between the active material layer andthe bonding layer, thereby improving adhesion with the layers. Theconductive material exists in the portion of interface between thecurrent collector and the bonding layer where the particulate polymerbinder does not exist and in the portion of the interface between theactive material layer and the bonding layer, so that exchange ofelectrons is carried out smoothly in the interface because of thepresence of the conductive material, and the electrical resistance canbe maintained at a low level. Moreover, since the particulate polymerbinder exists in the bonding layer, coagulating force in the bondinglayer increases and the cycle capacity maintaining characteristics ofthe battery is improved.

The main component of the polymer binder is preferably a fluororesin.

By using the fluororesin as the main component of the polymer binder, anelectrode for a secondary battery having high durability to theelectrolytic solution can be obtained.

The polymer binder is preferably a compound obtained by graftpolymerization of polyvinylidene fluoride and acrylic acid ormethacrylic acid as a monomer.

By using acrylic acid or methacrylic acid as the modifying material, theelectrode for a secondary battery containing the bonding layer havinggood adhesion with the current collector can be obtained.

The surface density of the particulate polymer binder in a cross sectionof the bonding layer parallel to the surface of the bonding layer ispreferably from 1 to 100/cm².

By controlling the surface density of the particulate polymer binder ina range from 1 to 100/cm², the particulate polymer binder is distributedwith a proper density in the interface between the current collector andthe bonding layer and the interface between the active material layerand the bonding layer, and both adhesion and electrical conductivity inthe interface can be maintained.

When the surface density is higher than 100/cm², electrical conductivityin the interface described above decreases. When the surface density islower than 1/cm², adhesion in the interface described above is reduced.The surface density is more preferably from 10 to 80/cm².

A sixth aspect of the present invention is a secondary batterycomprising the electrode for a secondary battery of the fifth aspect.

The secondary battery is excellent in cycle capacity maintainingcharacteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial partial schematic cross-sectional view showing anelectrode body of a lithium ion polymer secondary battery of the presentinvention.

FIG. 2 is a graph showing the results of an evaluation test (2) of thirdbinders obtained in Examples 5 to 11 and Comparative Examples 5 to 6.

FIG. 3 is a graph showing the results of an evaluation test (4) of thirdbinders obtained in Examples 5 to 11 and Comparative Examples 5 to 6.

FIG. 4 is a graph showing the results of an evaluation test (2) of thirdbinders obtained in Examples 12 to 16 and Comparative Examples 7 to 8.

FIG. 5 is a graph showing the results of an evaluation test (4) of thirdbinders obtained in Examples 12 to 16 and Comparative Examples 7 to 8.

FIG. 6 is a cross-sectional view taken along line A-A in FIG. 7 showinga lithium ion polymer secondary battery in accordance with the thirdembodiment of the present invention.

FIG. 7 is a perspective view showing a lithium ion polymer secondarybattery in accordance with the third embodiment of the presentinvention.

FIG. 8 is a cross-sectional view taken along line B-B in FIG. 9 showinga lithium ion polymer secondary battery in accordance with the fourthembodiment of the present invention.

FIG. 9 is a perspective view showing a lithium ion polymer secondarybattery in accordance with the fourth embodiment of the presentinvention.

FIG. 10 is an electron micrograph showing binder particles obtained byapplying and drying a polymer solution of Example 38.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The lithium ion polymer secondary battery in accordance with the firstembodiment of the present invention is characterized in that first andsecond bonding layers respectively contain both of a third binder and aconductive material, and the third binder is a polymer compound obtainedby modifying a first or second binder with a modifying material.

The term “modification” as used herein means change of properties andalso means to impart properties of a modifying material and newproperties, which do not exist in a polymer compound and a modifyingmaterial, by modifying the polymer compound using the modifyingmaterial, in addition to intrinsic properties of the polymer compoundbefore modification.

Since the modified polymer compound includes the first or second binderin the active material layer as a main group, it exhibits high adhesionwith the active material layer. Also adhesion with the current collectoris remarkably improved by modifying with the modifying material havinghigh adhesion with the current collector as compared with the case ofusing the same binder as that in the active material layer. Therefore,peeling of the material layer from the current collector is suppressedand cycle characteristics are improved.

The modified polymer compound becomes chemically stable by modification,compared with the binder used in the active material layer, and thuspeeling of the active material layer from the current collector issuppressed without being dissolved in the electrolytic solution. For thesame reason, since the conductive material dispersed in the bondinglayer is maintained without falling in, good electrical conductivity ismaintained and the resulting battery is excellent in long-term storagestability and cycle characteristics. Since the current collector iscoated with a chemically stable layer, even when hydrofluoric acid isgenerated in the battery, the bonding layer serves as a protectivelayer, and thus corrosion of the current collector can be prevented.

As compared with the binder used in the active material layer, themodified polymer compound becomes thermally stable by modification, andthus the battery is not dissolved in a solvent in the battery even athigh temperature and deterioration of the battery can be suppressed. Themodified polymer compound becomes electrochemically stable bymodification as compared with the binder used in the active materiallayer and the positive electrode does not deteriorate under highpotential upon full charging, and thus stable adhesion force andelectrical conductivity are maintained. Since it is difficult for theelectrolytic solution to penetrate into the modified polymer compound,the electrolytic solution does not significantly adhere to the currentcollector and dissociation of the positive electrode current collectorupon full charging can be suppressed.

The procedure for production of the lithium ion polymer secondarybattery of the present invention will now be described.

First, as the second embodiment of the present invention, the polymercompound, which forms a binder, contained in a positive electrode activematerial layer or a negative electrode active material layer, ismodified with a modifying material and the resulting modified polymercompound is used as a third binder of first and second bonding layers.

It is required that the first and second bonding layers be chemically,electrochemically and thermally stable and, therefore, the polymercompound, which is contained in the first or second binder used in theactive material and is also used as a raw material of the modifiedpolymer compound, is preferably a polymer compound containing fluorinein the molecule. Examples of the fluorine-containing polymer compoundinclude polytetrafluoroethylene, polychlorotrifluoroethylene, PVdF,vinylidene fluoride-hexafluoropropylene copolymer, and polyvinylfluoride.

Examples of the technique of modifying the fluorine-containing polymercompound include graft polymerization and crosslinking. Examples of themodifying material used in the graft polymerization include compoundssuch as ethylene, styrene, butadiene, vinyl chloride, vinyl acetate,acrylic acid, methyl acrylate, methyl vinyl ketone, acrylamide,acrylonitrile, vinylidene chloride, methacrylic acid, and methylmethacrylate. When using acrylic acid, methyl acrylate, methacrylic acidand methyl methacrylate, particularly good adhesion with the currentcollector can be obtained.

Examples of the modifying material used in the crosslinking includecompounds having two or more unsaturated bonds, for example, butadieneand isoprene. The crosslinking may be conducted by vulcanization.

As an example of this embodiment, graft polymerization will bedescribed. Examples of the method of graft polymerization include acatalytic method, chain transfer method, radiation method,photopolymerization method, and mechanical cutting method. In theradiation method, a polymer compound and a compound, which serves as agrafted material, can be polymerized by irradiating with radiationcontinuously or intermittently and the polymer compound as the maincomponent is preferably pre-irradiated before contacting the graftedmaterial with the polymer compound. Specifically, a modified polymercompound comprising the polymer compound as a main chain and a modifyingmaterial as a side chain can be obtained by irradiating the polymercompound with radiation and mixing the irradiated polymer compound withthe modifying material, which serves as the grafted material. Examplesof the radiation used in the graft polymerization include electron beam,X-rays, and γ-rays. The polymer compound is irradiated with γ-rays at anadsorption dose of 1 to 120 kGy. By irradiating the polymer compound, asthe main component, with radiation, a radical formed at one end and thegrafted material is easily polymerized. In the following chemicalschemes (1) and (2), graft polymerization of PVdF and acrylic acid by aradiation method is shown.

As shown in the chemical scheme (1), a radical is formed in the moleculeof PVdF by irradiating PVdF with γ-rays as radiation. As shown in thechemical scheme (2), PVdF having a radical in the molecule is contactedwith acrylic acid, whereby, a double bond portion of acrylic acid isgraft-polymerized by the radical of PVdF.

As another example, graft polymerization of PVdF and methacrylic acid isshown in the chemical schemes (3) and (4).

As shown in the chemical scheme (3), a radical is formed in the moleculeof PVdF by irradiating PVdF with γ-rays as radiation and PVdF having aradical in the molecule is contacted with methacrylic acid in thechemical scheme (4), whereby, a double bond portion of methacrylic acidis graft-polymerized by the radical of PVdF.

In the graft polymerization, a grafted product varies depending on thetime of contacting an activated polymer main chain with a monomer to begrafted, the degree of pre-activation of the polymer main chain byradiation, the monomer's ability of penetrating through the polymer mainchain, the kind of polymer to be grafted, and the temperature uponcontact with the monomer. In the case in which the monomer to be graftedis an acid, the proceeding degree of the graft polymerization reactioncan be observed by sampling a graft reaction solution containing amonomer at any time and measuring the concentration of the residualmonomer through titration with an alkali. The graft ratio in theresulting composition is preferably from 10 to 30% based on a finalweight.

Using the graft-polymerized modified polymer compound thus obtained as athird binder of the bonding layer, the third binder is dissolved in asolvent to prepare a polymer solution, and then a conductive material isdispersed in the polymer solution to prepare slurries for first andsecond bonding layers. As the conductive material, a carbon materialhaving a particle size of 0.5 to 30 μm and a graphitization degree of50% or more is used. A slurry for a bonding layer is prepared by mixinga third binder with a conductive material in a weight ratio (thirdbinder/conductive material) of 13/87 to 50/50. As the solvent,dimethylacetamide (hereinafter referred to as DMA), acetone,dimethylformamide and N-methyl pyrrolidone are used.

After a sheet-like positive electrode and a negative electrode currentcollector are prepared, the resulting slurries for first and secondbonding layers are respectively applied on positive electrode andnegative electrode current collectors by a doctor blade method and arethen dried to form positive electrode and negative electrode currentcollectors with first and second bonding layers each having a drythickness of 0.5 to 30 μm. The dry thickness of the bonding layer ofpositive and negative electrodes is preferably from 1 to 15 μm. Thesheet-like positive electrode current collector include Al foil and thenegative electrode current collector include Cu foil. The term “doctorblade method” as used herein is a method of controlling the thickness ofa slip to be carried on a carrier such as carrier film or endless beltby adjusting a distance between a knife edge referred to as a doctorblade and a carrier, thereby to precisely control the thickness of asheet.

The components required to form the positive electrode active materiallayer, the negative electrode active material layer and the electrolytelayer are mixed to prepare a coating slurry for a positive electrodeactive material layer, a coating slurry for a negative electrode activematerial layer and a coating slurry for an electrolyte layer.

The resulting coating slurry for a positive electrode active materiallayer is applied on the surface of a positive electrode currentcollector having a first bonding layer by a doctor blade method, isdried, and is then rolled to form a positive electrode. Similarly, thecoating slurry for a negative electrode active material layer is appliedon the surface of the surface of a negative electrode current collectorhaving a second bonding layer by a doctor blade method, is dried, and isthen rolled to form a negative electrode. The positive or negativeelectrode active material layer is formed in a dry thickness of 2 to 250μm. The resulting coating slurry for an electrolyte layer is applied ona release paper by a doctor blade method to form an electrolyte layerhaving a dry thickness of 1 to 150 μm. The resulting coating slurry foran electrolyte layer may be applied on the surface of the positiveelectrode and the surface of the negative electrode to form anelectrolyte layer. The resulting positive electrode, electrolyte layer,and negative electrode are laminated with each other, in this order, andthe resulting laminate is subjected to thermal compression bonding toform a sheet-like electrode body, as shown in FIG. 1.

Finally, a positive electrode lead and a negative electrode lead, eachmade of Ni, are respectively connected to a positive electrode currentcollector and a negative electrode current collector, and then theresulting electrode bodies were housed in a laminate packaging materialformed into a bag having an opening portion and the opening portion wassealed by thermal compression bonding under reduced pressure to obtain asheet-like lithium ion polymer secondary battery.

The third embodiment of the present invention will now be described withreference to the accompanying drawings.

As shown in FIG. 6, a lithium ion polymer secondary battery 110comprises a positive electrode 113 formed by providing a positiveelectrode active material layer containing a binder for a positiveelectrode and a positive electrode active material on the surface of apositive electrode current collector layer 112; a negative electrode 118formed by providing a negative electrode active material layer 116containing a binder for a negative electrode and a negative electrodeactive material on the surface of a negative electrode current collectorlayer 117; and a polymer electrolyte layer 121 interposed between thesurface of a positive electrode active material layer 111 of a positiveelectrode 113 and the surface of the negative electrode active materiallayer 116 of a negative electrode 118.

The positive electrode current collector layer 112 is made of an Al foiland the positive electrode active material layer 111 contains a positiveelectrode active material and a binder for a positive electrode. As thepositive electrode active material, powders of LiCoO₂, LiNiO₂ and LiMnO₄are used. The negative electrode current collector layer 117 is made ofa Cu foil and the negative electrode active material layer 116 containsa negative electrode active material and a binder for a negativeelectrode. As the negative electrode active material, powders of acarbon material such as graphite are used.

Since it is required that the binder for a positive electrode and thebinder for a negative electrode be chemically, electrochemically andthermally stable, a main component of the binder for a positiveelectrode and the binder for a negative electrode is preferably apolymer compound containing fluorine in the molecule. Examples of thefluorine-containing polymer compound include polytetrafluoroethylene,polychlorotrifluoroethylene, polyvinylidene fluoride, vinylidenefluoride-hexafluoropropylene copolymer, and polyvinyl fluoride.

As the polymer electrolyte layer 121, there can be used polymer sheets(for example, sheets made of fluororesins such as polyvinylidenefluoride and polyvinylidene fluoride-hexafluoropropylene copolymer, andpolymers such as polyethylene oxide) which contain an electrolyticsolution prepared by dissolving a lithium salt (for example, LiPF₆ orLiBF₄) in an organic solvent (for example, ethylene carbonate, propylenecarbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate,or γ-butyrolactone).

Between the positive electrode current collector layer 112 and thepositive electrode active material layer 111, a first bonding layer 114countering a first binder and a first conductive material is interposed.Between the negative electrode current collector layer 117 and thenegative electrode active material layer 116, a second bonding layer 119containing a second binder and a second conductive material isinterposed. The first bonding layer 114 is formed by dispersing thefirst conductive material in the first binder. Similarly, the secondbonding layer 119 is formed by dispersing the second conductive materialin the second binder. Since the first bonding layer 114 or the secondbonding layer 119 are chemically, electrochemically and thermallystable, and adhesion with the current collector layer and the activematerial layer is required, the binder for a positive electrode and thebinder for a negative electrode, which are respectively contained in thepositive electrode active material layer and the negative electrodeactive material layer, are used as main components of the first andsecond binders. As the first and second conductive materials, metal orpartially oxidized metal each having a particle size of 0.1 to 20 μm isused. The particle size of these first and second conductive materialsis from 0.1 to 20 μm, and preferably from 0.3 to 15 μm. When theparticle size is less than 0.1 μm, particles agglomerate, and it becomesimpossible to sufficiently disperse the conductive material on thecurrent collector, resulting in low electronical conductivity and pooroutput characteristics. Since a proportion of the binder decreases atthe agglomerated portion, adhesion force decreases, and also, cyclecharacteristics deteriorate. On the other hand, when the size exceeds 20μm, the thickness of the bonding layer increases and thus volume energydensity decreases. Since it becomes difficult to contact the materials,electronic conductivity is reduced, and also, output characteristicsdeteriorate. Examples of the metal include aluminum, copper, iron,nickel, cobalt, silver, gold, platinum, and palladium, and examples ofthe partially oxidized metal include oxides obtained by oxidizing aportion of the above-mentioned metals. As the first and secondconductive materials, mixtures or alloys of one, or two, or more kindsselected from the group consisting of the above-mentioned metals andpartially oxidized metals. By using metal or partially oxidized metal asthe first and second conductive materials, good electrical conductivitycan be obtained by adding a small amount of the conductive material, andthus the volume of the conductive material can be remarkably reduced.

The third embodiment of the present invention is characterized in thatthe first binder and the first conductive material contained in thefirst bonding layer 114, and the second binder and the second conductivematerial contained in the second bonding layer 119 are respectivelymixed in a weight ratio of 13/87 to 75/25. By controlling the weightratio of the first binder to the first conductive material contained inthe first bonding layer 114 and the weight ratio of the second binder tothe second conductive material contained in the second bonding layer 119in the above-mentioned ranges, it is made possible to obtain a lithiumion polymer secondary battery which is excellent in adhesion andelectrical conductivity between the positive electrode current collectorand the positive electrode active material layer or between the negativeelectrode current collector and the negative electrode active materiallayer, and also has improved cycle capacity maintaining characteristics.The weight ratio of the first binder to the first conductive materialand the weight ratio of the second binder to the second conductivematerial are from 13/87 to 75/25, and preferably from 14/86 to 33/67.When the weight ratio is less than 13/87, the proportion of the binderis small and sufficient adhesion force cannot be obtained. When theweight ratio exceeds 75/25, the amount of the conductive materialcontained in the bonding layer is small and sufficient electron transferbetween the current collector and the active material layer cannot beconducted, and thus internal impedance increases.

A positive electrode 113 is formed in the following manner. A positiveelectrode current collector made of a sheet-like Al foil is prepared andthe slurry for a first bonding layer is applied on the positiveelectrode current collector 112 and is then dried to form a positiveelectrode current collector with a first bonding layer having a drythickness of 0.5 to 30 μm. The dry thickness of the first bonding layeris preferably from 1 to 15 μm. The components required to form apositive electrode active material layer are mixed to prepare a coatingslurry for a positive electrode active material layer. The resultingcoating slurry for a positive electrode active material layer is appliedon the surface of the positive electrode current collector having afirst bonding layer, is dried, and is then rolled to form a positiveelectrode 113. The positive electrode active material layer is formed sothat the dry thickness is from 20 to 250 μm.

A negative electrode 118 is formed in the following manner. A negativeelectrode current collector made of a sheet-like Cu foil is prepared andthe slurry for a second bonding layer is applied on the negativeelectrode current collector and is then dried to form a negativeelectrode current collector with a second bonding layer having a drythickness of 0.5 to 30 μm. The dry thickness of the second bonding layerof the negative electrode is preferably from 1 to 15 μm. The componentsrequired to form a negative electrode active material layer are mixed toprepare a coating slurry for a negative electrode active material layer.In the same manner as in the case of forming the positive electrode, theresulting coating slurry for a negative electrode active material layeris applied on the surface of the negative electrode current collectorhaving a second bonding layer, is dried, and is then rolled to form anegative electrode. The negative electrode active material layer isformed so that the dry thickness is from 20 to 250 μm.

A polymer electrolyte layer 121 is formed in the following manner. Thecomponents required to form the polymer electrolyte layer are mixed toprepare a coating slurry for a polymer electrolyte layer. The resultingcoating slurry for a polymer electrolyte layer is applied on a releasepaper so that the dry thickness of the polymer electrolyte layer becomes10 to 150 μm, and it is then dried to form a polymer electrolyte layer.

A laminate is obtained by folding the polymer electrolyte layer 121 froma center, laminating to cover both surfaces of the edge portion of thenegative electrode 118, and laminating the positive electrode 113 on thepartial laminate. Then, a rolled body 122 is made by winding theresulting laminate in the form of a flattened roll (FIG. 6 and FIG. 7).As shown in FIG. 7, a single sheet-like positive electrode terminal 123made of Ni, one end of which is connected electrically to the positiveelectrode current collector layer 112 of the rolled body 122, protrudesfrom one edge 122 a of the rolled body 122, while a single sheet-likepositive electrode terminal 124 made of Ni, one end of which isconnected electrically to the negative electrode current collector layer117 of the rolled body 122, protrudes from the other edge 122 b of therolled body 122. Furthermore, the rolled body 122 is enclosed in apackage 125 (FIG. 6 and FIG. 7) so as to protrude from the other end ofthe positive electrode terminal 123 and the other end of the negativeelectrode terminal 124. The rolled body 122 is housed in a laminatepackaging material 125 formed into a bag having an opening portion so asto protrude from the other end of the positive electrode terminal 123and from the other end of the negative electrode terminal 124, and thenthe opening portion was sealed by thermal compression bonding underreduced pressure to obtain a sheet-like lithium ion polymer secondarybattery.

The fourth embodiment of the present invention will now be described. InFIG. 8, the same symbols as in FIG. 6 denote the same parts.

In this embodiment, a positive electrode 113, a negative electrode 118,and a polymer electrolyte layer 121, which are formed in almost the samesize, are laminated to make a laminate which is housed in a afterpleatingdly folding the laminate. The phrase “pleatingly folding thelaminate” refers to folding the laminate in a zigzaging manner. Thisembodiment has the same constitution as that of the first embodiment,except in the above respect.

The method of producing a secondary battery using the positive electrode113 and the negative electrode 118 will now be described.

First, a negative electrode 118, a polymer electrolyte layer 121 and apositive electrode 113 are laminated to make a laminate. Then, thelaminate is pleatingly folded to make a flat pleatedly folded material152 (FIG. 8 and FIG. 9). As shown in FIG. 9, a single sheet-likepositive electrode terminal 123, one end of which is connectedelectrically to the positive electrode current collector layer 112 ofthe pleatedly folded material 152, protrudes from one edge 152 a of thepleatedly folded material 152, while a single sheet-like positiveelectrode terminal 124, one end of which is connected electrically tothe negative electrode current collector layer 117 of the pleatedlyfolded material 152, protrudes from the other edge of the pleatedlyfolded material 152. Furthermore, the pleatedly folded material 152 isenclosed in a package 125 (FIG. 8 and FIG. 9) so as to protrude from theother end of the positive electrode terminal 123 and the other end ofthe negative electrode terminal 124.

In the lithium ion polymer secondary battery 150 with such aconstitution, the number man hours required to assemble the battery canbe reduced as compared with the battery of the first embodiment.

The fifth embodiment of the present invention will now be described.

As shown in FIG. 1, a lithium ion polymer secondary battery comprises anelectrode body 10 formed by laminating a positive electrode 11 and anegative electrode 14. The positive electrode 11 is made by providing apositive electrode active material layer 13 on the surface of asheet-like positive electrode current collector 12, while the negativeelectrode 14 is made by providing a negative electrode active materiallayer 17 on the surface of a sheet-like negative electrode currentcollector 16. In the state where an electrolyte layer 18 is interposedbetween the positive electrode active material layer 13 and the negativeelectrode active material layer 17, the positive electrode 11 and thenegative electrode 14 are laminated to form an electrode body 10. Thepositive electrode active material layer 13 contains a first binder, inaddition to an active material such as LiCoO₂, while the negativeelectrode active material layer 17 contains a first binder, in additionto an active material such as graphite.

A first bonding layer 19 is provided between the positive electrodecurrent collector 12 and the positive electrode active material layer13, while a second bonding layer 21 is provided between the negativeelectrode current collector 16 and the negative electrode activematerial layer 17. The first and second bonding layers 19 and 21 containboth a polymer binder and a conductive material. The polymer binder inthis embodiment is a polymer compound obtained by modifying the first orsecond binder with a modifying material. The term “modification” as usedherein has the same meaning as in the first embodiment. The electrodefor secondary battery of this embodiment is characterized in that aportion of the polymer binder exists in the bonding layers 19 and 21 inthe form of particles and the volume-mean particle size of theparticulate polymer binder is from 1 to 100 μm.

In the electrodes for battery 11 and 14 with such a constitution, theparticulate polymer binder that exists in the bonding layers 19 and 21exists together with the conductive material, that exists in the form ofparticles, in the interface between the current collectors 12 and 16 andthe bonding layers 19 and 21 and in the interface between the activematerial layers 13 and 17 and the bonding layers 19 and 21, therebyimproving adhesion with the layers. The conductive material exists inthe portion of interface between the current collectors 12 and 16 andthe bonding layers 19 and 21 where the particulate polymer binder doesnot exist and in the portion of the interface between the activematerial layers 13 and 17 and the bonding layers 19 and 21, so thatexchange of electrons is carried out smoothly in the interface becauseof the presence of the conductive material, and the electricalresistance can be maintained at a low level.

Also a portion of the polymer binder exists in the bonding layers 19 and21 in the form of particles, thereby improving the mechanical strengthof the bonding layers 19 and 21 improving the strength of the activematerial layers 13 and 17 which are laminated in contact with thebonding layers 19 and 21. Furthermore, it is not required that thebonding layers 19 and 21 in the present invention be formed in a patternof dots, stripes, or a grid. Therefore, it becomes easy to form thebonding layers 19 and 21 as compared with the case where the bondinglayers must be formed in a predetermined pattern.

The procedure for the production of an electrode for a secondary batteryof the fifth embodiment of the present invention will now be described.

First, the binder to be contained in the positive electrode activematerial layer or the negative electrode active material layer ismodified with a modifying material, and then the resulting modifiedpolymer compound is used as a polymer binder of the first and secondbonding layers.

Since it is required that the first and second bonding layers 19 and 21be chemically, electrochemically, and thermally stable, a polymercompound, which is a first or second binder used as an active materiallayer and is also a raw material of the modified polymer compound, ispreferably a polymer compound containing fluorine in the molecule.Examples of the fluorine-containing polymer compound includepolytetrafluoroethylene, polychlorotrifluoroethylene, PVdF, vinylidenefluoride-hexafluoropropylene copolymer, and polyvinyl fluoride.

Examples of the technique of modifying the fluorine-containing polymercompound include graft polymerization and crosslinking. Examples of themodifying material used in the graft polymerization include compoundssuch as ethylene, styrene, butadiene, vinyl chloride, vinyl acetate,acrylic acid, methyl acrylate, methyl vinyl ketone, acrylamide,acrylonitrile, vinylidene chloride, methacrylic acid, and methylmethacrylate. When using acrylic acid, methyl acrylate, methacrylicacid, and methyl methacrylate, particularly good adhesion with thecurrent collector can be obtained.

Examples of the modifying material used in the crosslinking includecompound having two or more unsaturated bonds, for example, butadieneand isoprene. The crosslinking may also be conducted by vulcanization.

Using the modified polymer compound thus obtained as a polymer binder ofthe bonding layer, the polymer binder is partially dissolved in asolvent to prepare a particulate polymer-containing polymer solution,and then a conductive material is dispersed in the resulting polymersolution to prepare slurries for first and second bonding layers. As theconductive material, a carbon material having a particle size of 0.5 to30 μm and a graphitization degree of 50% or more is used. The polymerbinder and the conductive material are mixed in a weight ratio (polymerbinder/conductive material) of 13/87 to 50/50 to prepare a slurry for abonding layer. As the solvent, dimethylacetamide (hereinafter referredto as DMA), acetone, dimethylformamide and N-methylpyrrolidone are used.

Sheet-like positive- and negative electrode current collectors 12 and 16are prepared and the slurry for first and second bonding layers wereapplied on the positive- and negative electrode current collectors 12and 16 by a doctor blade method to form positive- and negative electrodecurrent collectors 12 and 16 with first and second bonding layers 19 and21 each having a dry thickness of 0.5 to 30 μm. The dry thickness of thebonding layers 19 and 21 is preferably from 1 to 15 μm. Examples of thesheet-like positive electrode current collector 12 include Al foil, andexamples of the negative electrode current collector 16 include Cu foil.The term “doctor blade method” as used herein is a method of controllingthe thickness of a slip to be carried on a carrier such as a carrierfilm or an endless belt by adjusting a distance between a knife edge,referred to as a doctor blade, and a carrier, thereby preciselycontroling the thickness of a sheet.

Then, the components required to form a positive electrode activematerial layer 13 and a negative electrode active material layer 17 aremixed to prepare a coating slurry for a positive electrode activematerial layer and acoating slurry for a negative electrode activematerial layer. With respect to a positive electrode, the resultingcoating slurry for a positive electrode active material layer is appliedon the positive electrode current collector 12 having the first bondinglayer 19 by a doctor blade method, is dried, and is then rolled. Also,with respect to a negative electrode 14, the resulting coating slurryfor a negative electrode active material layer is applied on thenegative electrode current collector 16 having the second bonding layer21 by a doctor blade method, is dried, and is then rolled. The positive-and negative electrode active material layers 13 and 17 are formed sothat the dry thickness becomes 20 to 250 μm. As described above, thepositive electrode 11 for a secondary battery of the present inventionand the negative electrode 14 for a secondary battery of the presentinvention are formed.

A secondary battery including the electrodes as the fifth embodiment ofthe present invention in accordance with the sixth embodiment of thepresent invention will now be described.

The secondary battery including the electrodes as the fifth embodimentof the present invention comprises the above-mentioned positiveelectrode 11 for secondary battery and the negative electrode 14 forsecondary battery. Specific manufacturing procedures are as follows.First, the positive electrode 11 for a secondary battery and thenegative electrode 14 for a secondary battery are prepared. Then,components required for an electrolyte layer 18 are mixed to prepare aslurry for formation of the electrolyte layer. The resulting coatingslurry for an electrolyte layer is applied on a release paper by adoctor blade method so that the dry thickness of the electrolyte layer18 becomes 10 to 150 μm, and is then dried to form an electrolyte layer.Also, the electrolyte layer 18 may be formed by applying the coatingslurry for an electrolyte layer on the surface on a positive electrode11 or the surface of a negative electrode 14 and then drying. Thepositive electrode 11, the electrolyte layer 18 and the negativeelectrode 14 are laminated with each other, in this order, and theresulting laminate is subjected to thermal compression bonding to form asheet-like electrode body 10 shown in FIG. 1.

A lithium ion polymer secondary battery of the present invention can beproduced by respectively connecting a positive electrode lead and anegative electrode lead, each made of Ni (not shown), to a positiveelectrode current collector 12 and a negative electrode currentcollector 16, housing the sheet-like electrode bodies 10 in a laminatepackaging material formed into a bag having an opening portion, andsealing the opening portion by thermal compression bonding under reducedpressure.

Examples of the present invention will now be described, together withComparative Examples.

EXAMPLE 1

First, 50 g of powdered PVdF was prepared as first and second bindersand 260 g of an aqueous 15 wt % acrylic acid solution was prepared as amodifying material. The powdered PVdF was placed in a polyethylene pack,vacuum-packed, and then irradiated with γ-rays at an absorption dose of50 kGy using cobalt-60 as the γ-ray source. The powdered PVdF irradiatedwith γ-rays was taken out from the polyethylene pack and transferredinto a nitrogen atmosphere. PVdF was supplied into 260 g of the aqueous15 wt % acrylic acid solution, maintained at 80° C. and then reactedwith the aqueous acrylic acid solution, thereby to synthesize acrylicacid grafting polyvinylidene fluoride produced by the graftpolymerization shown in the above chemical scheme (2) (hereinafterreferred to as AA-g-PVdF).

A sample of the reaction solution was collected and a decrease in amountof the acrylic acid due to the graft polymerization reaction with PVdFwas successively measured by titration. When the content of the gratedacrylic acid group in AA-g-PVdF reached 17% by weight, the reaction wasterminated and the resulting solid product was washed with pure waterand was then dried to obtain a third binder.

EXAMPLE 2

In the same manner as in Example 1, except that the modifying materialwas replaced by 260 g of an aqueous 15 wt % acrylic acid solution, athird binder was obtained.

EXAMPLE 3

In the same manner as in Example 1, except that the modifying materialwas replaced by 260 g of an aqueous 15 wt % methyl acrylate solution, athird binder was obtained.

EXAMPLE 4

In the same manner as in Example 1, except that the modifying materialwas replaced by 260 g of an aqueous 15 wt % methyl methacrylatesolution, a third binder was obtained.

COMPARATIVE EXAMPLE 1

A commercially available acrylate ester-methacrylate ester copolymer wasprepared as a third binder.

COMPARATIVE EXAMPLE 2

A commercially available homopolymer of PVdF was prepared as a thirdbinder.

COMPARATIVE EXAMPLE 3

A commercially available copolymer of PVdF-HFP was prepared as a thirdbinder.

COMPARATIVE EXAMPLE 4

In the same manner as in Example 1, except that the modifying materialwas replaced by 2600 g of an aqueous solution containing 1% by weight ofcrotonic acid, a third binder was obtained.

Comparative Evaluation 1

Using the third binders obtained in Examples 1 to 4 and ComparativeExamples 1 to 4, the following evaluation tests were conducted.

(1) Test for Solubility in Dimethylacetamide

Polymer solutions were prepared by weighing 4 g of each of the thirdbinders obtained in Examples 1 to 4 and Comparative Examples 1 to 4, andadding 56 g of DMA to samples, followed by stirring with heating to 60°C. The resulting solutions were stored in a glass bottle and wereallowed to stand for one day, and then the state of precipitation in thesolutions was determined.

(2) Test for Adhesion with Copper Foil and Aluminum Foil

First, the same polymer solutions as those for the above-mentionedevaluation test (1) were prepared by using the third binders obtained inExamples 1 to 4 and Comparative Examples 1 to 4. Each of these solutionswas uniformly applied on a Cu foil having a width of 30 mm, a length of200 mm and a thickness of 14 μm, the surface of which was degreased, andthen an Al foil having a width of 10 mm, a length of 100 mm and athickness of 20 μm, the surface of which was degreased, was appliedthereon to make a peel specimen for adhesion test. The resultingspecimen was dried in atmospheric air at 80° C. for 5 days using adryer. Then, the binder of the dried specimen to the Cu foil and the Alfoil was evaluated by a machine for peeling testing. In the peelingtest, the Cu foil side of the specimen was fixed to a testing stand andthe Al foil bonded to the Cu foil was vertically pulled upward at a rateof 100 mm/min, and then a force required to peel off the Al foil fromthe Cu foil was measured.

(3) Test for Cycle Capacity Maintaining Characteristics of Battery WhenUsed in the Bonding Layer of a Battery

First, polymer solutions were prepared by weighing 2 g of each of thethird binders obtained in Examples 1 to 4 and Comparative Examples 1 to4, and adding 200 g of DMA to the samples, followed by stirring andheating to 60° C. To each of the solutions, 8 g of powdered graphitehaving a specific surface area of 150 m²/g and 1.2 g of a dispersant fordispersing the powdered graphite were added to prepare a slurry for abonding layer. After preparing an Al foil having a thickness of 20 μmand a width of 250 μm as a positive electrode current collector, theresulting slurry for a bonding layer was applied on the Al foil by adoctor blade method and was then dried. The thickness of the bondinglayer after drying was controlled within a range of 10±1 μm. Afterpreparing a Cu foil having a thickness of 14 μm and a width of 250 μm asa negative electrode current collector, the resulting slurry for abonding layer was applied on the Cu foil by a doctor blade method andwas then dried. The thickness of the bonding layer after drying wascontrolled within a range of 10±1 μm. The respective components shown inTable 1 below were mixed in a ball mill for 2 hours to prepare a coatingslurry for a positive electrode active material layer, a coating slurryfor a negative electrode active material layer, and a coating slurry foran electrolyte layer.

TABLE 1 Coating slurry Parts component by weight Positive LiCoO₂ 90electrode active Powdered graphite 6 material layer PVdF 4 N-methylpyrrolidone 45 Negative Powdered graphite 90 electrode active PVdF 10material layer N-methyl pyrrolidone 50 Electrolyte layer Vinylidene 17fluoride-hexafluoropropylene copolymer Propylene carbonate 15 Ethylenecarbonate 30 Diethyl carbonate 30 LiPF₆ 8 Acetone 80

The resulting coating slurry for a positive electrode active materiallayer was applied on the surface of the Al foil having the bonding layerby a doctor blade method so that the dry thickness of the positiveelectrode active material layer was 80 μm, was dried, and was thenrolled to form a positive electrode. Similarly, the coating slurry for anegative electrode active material layer was applied on the surface ofthe Cu foil having the bonding layer by a doctor blade method so thatthe dry thickness of the positive electrode active material layer was 80μm, was dried, and wasthen rolled to form a negative electrode.Furthermore, the coating slurry for an electrolyte layer wasrespectively applied on the positive electrode and the negativeelectrode by a doctor blade method so that each dry thickness was 50 μm,and then the positive and negative electrodes, each having anelectrolyte layer, were laminated with each other by thermal compressionbonding to yield sheet-like electrode bodies. In the resultingsheet-like electrode bodies, a positive electrode lead and a negativeelectrode lead, each being made of Ni, were respectively connected to apositive electrode current collector and a negative electrode currentcollector, and then the sheet-like electrode bodies were housed in alaminate packaging material formed into a bag having an opening portion,and the opening portion was sealed by thermal compression bonding underreduced pressure to obtain a sheet-like battery.

Then, a charge-discharge cycle, comprising a charge step of charging theresulting sheet-like battery under the conditions of a maximum chargevoltage of 4 V and a charge current of 0.5 A for 2.5 hours, and adischarge step of discharging at a constant current of 0.5 A until thedischarge voltage was reduced to a minimum discharge voltage of 2.75 V,was repeated, and then a charge-discharge capacity of each cycle wasmeasured and the number of cycles required to reduce to 80% of aninitial discharge capacity was measured.

(4) Test for Adhesion of Bonding Layer to Current Collector When Used inBonding Layer of Battery

First, the same sheet-like batteries as those for the above-mentionedevaluation test (3) were prepared by using the third binders obtained inExamples 1 to 4 and Comparative Examples 1 to 4. Under conditions of 70°C., 100 charge-discharge cycles of the resulting batteries wereconducted under the same conditions as those in the evaluation test (3).After the completion of 100 charge-discharge cycles, the housing packageof the sheet-like battery was removed and the positive electrode and thenegative electrode of the battery were detached and separated. It wasthen determined whether or not the bonding layer would peeled off fromthe current collector when pulling the bonding layers of the separatedpositive and negative electrodes using forceps.

The evaluation results in the evaluation tests (1) to (4) are shown inTable 2. Symbols in the column of the evaluation test (1) in Table 2have the following meaning.

-   ⊚: uniform solution with no precipitate.

Symbols in the column of the evaluation test (4) shown in Table 2 havethe following meanings.

-   ⊚: excellent adhesion of bonding layer to current collector; not    peeled off-   ◯: bonding layer is partially peeled off from current collector-   Δ: bonding layer is peeled off from current collector over a large    area-   ×: bonding layer is completely peeled off from current collector

TABLE 2 Evaluation Evaluation Evaluation Evaluation test (4) test (1)test (2) test (3) 80% Adhesion of bonding layer to current Solubility ofPeel strength Capacity cycle collector after 100 cycles third binder[N/cm] of Cu number [times] of Positive electrode Negative electrode inDMA and Al bonded battery current collector current collector Example 1⊚ 16.46 635 ⊚ ⊚ Example 2 ⊚ 10.09 480 ⊚ ◯ Example 3 ⊚ 15.58 565 ⊚ ⊚Example 4 ⊚ 13.15 523 ⊚ ⊚ Comparative ⊚ 6.89 194 X Δ Example 1Comparative ⊚ 0.98 183 X X Example 2 Comparative ⊚ 0.98 157 X X Example3 Comparative ⊚ 1.96 92 Δ Δ Example 4

As is apparent from Table 2, the solubility in DMA as in the evaluationtest (1) is suitable for use as a coating slurry because the thirdbinders obtained in Examples 1 to 4 and Comparative Examples 1 to 4 canbe completely dissolved in DMA and no precipitate is produced even whenthe solution is allowed to stand for one day.

In the test for adhesion with the Cu foil and the Al foil as theevaluation test (2), the peel strength was 2 N/cm or less in the testusing the third binders obtained in Comparative Examples 2 to 4 and,therefore, it was found that the bonding effect as the bonding layermaterial was insufficient. In contrast, the Al foil bonded to the Cufoil using the third binders obtained in Examples 1 to 4 exhibited astrength sufficient to bond the active material with the currentcollector of the battery because all peel strengths from the Cu foilwere 10 N/cm or more.

In the cycle capacity maintaining characteristics test as the evaluationtest (3), 80% capacity cycle number of the batteries using the thirdbinders of Examples 1 to 4 were higher than those of batteries using thethird binders of Comparative Examples 1 to 4. The reason is believed tobe as follows. That is, cycle capacity maintaining characteristics wereimproved because the third binders of Examples 1 to 4 are excellent inadhesion and have high durability to the electrolytic solution.

In the test for adhesion of the bonding layer to the current collectoras the evaluation test (4), the bonding layers using the third bindersof Examples 1 to 4 are excellent in adhesion with the current collectorbecause they are not easily peeled off from the current collector ascompared with the bonding layers using the third binders of ComparativeExamples 1 to 4. The batteries of Comparative Example 1 using acopolymer containing no fluorine as the third binder are excellent inadhesion with the Cu foil and the Al foil, but are insufficient inresistance to the electrolytic solution. Therefore, after repeatedcharge-discharge cycles, the bonding layer was peeled off from thecurrent collector of the battery.

As is apparent from the results of the above evaluation tests, amodified polymer compound obtained by grafting PVdF with a modifyingmaterial, particularly acrylic acid, methyl acrylate, methacrylic acid,or methyl methacrylate is, suitable for use as the third binder of thefirst and second bonding layers.

EXAMPLES 5 TO 11 AND COMPARATIVE EXAMPLES 5 TO 6

First, 50 g of powdered PVdF was prepared as first and second bindersand 260 g of an aqueous 15 wt % acrylic acid solution was prepared as amodifying material. The powdered PVdF was placed in a polyethylene pack,vacuum-packed, and then irradiated with γ-rays at an absorption dose of50 kGy using cobalt-60 as the γ-ray source. The powdered PVdF irradiatedwith γ-rays was taken out from the polyethylene pack and transferredinto a nitrogen atmosphere. PVdF was supplied into 260 g of the aqueous15 wt % acrylic acid solution, maintained at 80° C., and then reactedwith the aqueous acrylic acid solution, thereby synthesizing AA-g-PVdF.

A sample of the reaction solution was collected, and a decrease in theamount of the acrylic acid due to the graft polymerization reaction withPVdF was successively measured by titration. When the content of thegrated acrylic acid group in AA-g-PVdF reached 2% by weight (Example 5),7% by weight (Example 6), 10% by weight (Example 7), 13% by weight(Example 8), 17% by weight (Example 9), 25% by weight (Example 10), 40%by weight (Example 11), 1% by weight (Comparative Example 5), or 55% byweight (Comparative Example 6), the reaction was terminated, and theresulting solid product was washed with pure water and was then dried toobtain the third binders of Examples 5 to 11 and Comparative Examples 5to 6.

Comparative Evaluation 2

Using the third binders obtained in Examples 5 to 11 and ComparativeExamples 5 to 6, the same evaluation tests (1) to (4) as in comparativeevaluation 1 were conducted and the influence of the content of anacrylic acid group in nine kinds of AA-g-PVdF on the adhesion andelectrical characteristics of the battery was examined. The results ofthe evaluation tests (1) and (4) are shown in Table 3, the results ofthe evaluation test (2) are shown in FIG. 2 and the results of theevaluation test (3) are shown in FIG. 3. Symbols in the column of theevaluation test (1) shown in Table 3 have the following meanings.

-   ⊚: uniform solution with no precipitate-   ◯: dissolved in DMA; a small amount of precipitate is produced-   ×: not dissolved in DMA

Symbols in the column of the evaluation test (4) shown in Table 3 havethe following meanings.

-   ⊚: bonding layer is not peeled off because of excellent adhesion    with current collector-   ◯: bonding layer is partially peeled off from current collector-   ×: bonding layer is completely peeled off from current collector

TABLE 3 Evaluation Evaluation test (4) test (1) Adhesion of bondinglayer to current Solubility collector after 110 cycles of third binderPositive electrode Negative electrode in DMA current collector currentcollector Example 5 ⊚ ◯ ◯ Example 6 ⊚ ⊚ ⊚ Example 7 ⊚ ⊚ ⊚ Example 8 ⊚ ⊚⊚ Example 9 ⊚ ⊚ ⊚ Example 10 ⊚ ⊚ ⊚ Example 11 ◯ ⊚ ◯ Comparative ⊚ X XExample 5 Comparative X X X Example 6

As is apparent from Table 3, the third binders of Examples 5 to 9 havinga content of an acrylic acid group of 25% by weight or less werecompletely dissolved in DMA, and no precipitate was produced even whenallowed to stand, regarding the solubility in DMA as the evaluation test(1). The third binder of Example 10 having a content of an acrylic acidgroup of 25% by weight was not easily dissolved in DMA and a smallamount of a precipitate was produced when the solution was allowed tostand for one day. A small amount of the precipitate is dissolved againby stirring the solution for a long time using a homogenizer and is usedto adhere to the copper foil and the aluminum foil without causing anyproblem. In contrast, the third binder of Example 6 having a content ofan acrylic acid group of 55% by weight scarcely dissolved in DMA and isnot suitable to adhere to the foil and the aluminum foil. The thirdbinder of Comparative Example 5 having a content of an acrylic acidgroup of 1% by weight was dissolved in DMA without causing any problem.

In the test for adhesion with the Cu foil and the Al copper foil as theevaluation test (2), as shown in FIG. 2, the adhesive strength of the Cufoil and the Al foil bonded with AA-g-PVdF has a correlation with thecontent of an acrylic acid group in AA-g-PVdF. As is apparent from theresults of Comparative Example 6, the effect of bonding is reduced whenthe content becomes 55% by weight or more. This is because thesolubility of AA-g-PVdF in DMA was insufficient, as is apparent from theresults of the evaluation test (1). Also in Comparative Example 5wherein the content of the acrylic acid group is 1% by weight, theadhesive strength decreased. As is apparent from the results of Examples5 to 11, the content of the acrylic acid group in AA-g-PVdF must bewithin a range from 2 to 50% by weight, and preferably from 10 to 30% byweight, in order to obtain sufficient adhesion.

In the test for cycle capacity maintaining characteristics as theevaluation test (3), as shown in FIG. 3, the test results are the sameas those in the adhesive strength as the evaluation test (2).

In the test for adhesion of the bonding layer to the current collectoras the evaluation test (4), as is apparent from Table 3, when thecontent of the acrylic acid group in AA-g-PVdF is from 2 to 50% byweight, and preferably from 10 to 30% by weight, adhesioncharacteristics of the bonding layer are suited for use as the binder ofthe battery similar to the evaluation test (2) and the evaluation test(3) as a result of analysis of the bonding layer of the battery afterrepeating charge-discharge cycles.

EXAMPLES 12 TO 16 AND COMPARATIVE EXAMPLES 7 TO 8

First, 50 g of powdered PVdF was prepared as first and second bindersand 260 g of an aqueous 15 wt % acrylic acid solution was prepared as amodifying material. The powdered PVdF was put in a polyethylene pack,vacuum-packed, and then irradiated with γ-rays at an absorption dose of90 kGy (Example 12), 70 kGy (Example 13), 50 kGy (Example 14), 20 kGy(Example 15), 10 kGy (Example 16), 130 kGy (Comparative Example 7) or0.5 kGy (Comparative Example 8) using cobalt-60 as a γ-ray source. Thepowdered PVdF irradiated with γ-ray was taken out from the polyethylenepack and transferred into a nitrogen atmosphere. PVdF was supplied into260 g of the aqueous 15 wt % acrylic acid solution, maintained at 80°C., and then reacted with the aqueous acrylic acid solution, therebysynthesizing AA-g-PVdF.

A sample of the reaction solution was collected and a decrease in amountof the acrylic acid due to the graft polymerization reaction with PVdFwas successively measured by titration. When the content of the gratedacrylic acid group in AA-g-PVdF reached 17% by weight, the reaction wasterminated and the resulting solid product was washed with pure waterand then dried to obtain third binders of Examples 12 to 16 andComparative Examples 7 to 8.

Comparative Evaluation 3

Using the third binders obtained in Examples 12 to 16 and ComparativeExamples 7 to 8, the same evaluation tests (1) to (4) as in comparativeevaluation 1 were conducted. The results of the evaluation tests (1) and(4) are shown in Table 4, the results of the evaluation test (2) areshown in FIG. 4, and the results of the evaluation test (3) are shown inFIG. 5. Symbols in the columns of the evaluation tests (1) and (4) shownin Table 4 have the same meanings as those used in the comparativeevaluation 2.

TABLE 4 Evaluation Evaluation test (4) test (1) Adhesion of bondinglayer to current Solubility collector after 100 cycles of third binderPositive electrode Negative electrode in DMA current collector currentcollector Example 12 ◯ ⊚ ◯ Example 13 ⊚ ⊚ ⊚ Example 14 ⊚ ⊚ ⊚ Example 15⊚ ⊚ ⊚ Example 16 ⊚ ◯ ◯ Comparative X X X Example 7 Comparative ⊚ X XExample 8

As is apparent from Table 4, regarding the solubility in DMA as theevaluation test (1), in Examples 13 to 16, in which the absorption dosewas less than 70 kGy, the synthesized AA-g-PVdF could be dissolved inDMA even at room temperature. In Example 12, in which the absorptiondose was 90 kGy, the synthesized AA-g-PVdF was dissolved in DMAmaintained at high temperature (85° C.). In contrast, in ComparativeExample 7, in which the absorption dose was 130 kGy, it was nearlyimpossible to dissolve in DMA even when stirred.

In the test for adhesion with the Cu foil and the Al foil as theevaluation test (2), as shown in FIG. 4, the adhesive strength of the Cufoil and the Al foil bonded with AA-g-PVdF has a correlation with thecontent of an acrylic acid group in AA-g-PVdF. As is apparent from theresults of Comparative Examples 7 and 8, when the absorption dose isless than 1 kGy or exceeds 120 kGy, the adhesive strength of AA-g-PVdFis reduced and the resulting binder is not suited for use as the binder.The reason is believed to be as follows. That is, when the dose is lessthan 1 kGy, fewer acrylic acid groups are grafted, and when the doseexceeds 120 kGy, AA-g-PVdF has insufficient solubility.

In the test for cycle capacity maintaining characteristics as theevaluation test (3), as shown in FIG. 5, the test results are the sameas those in the adhesive strength as the evaluation test (2). As isapparent from these results, the absorption dose is preferably within arange from 1 kGy to 120 kGy in order to produce a battery capable ofmaintaining 80% capacity at 400 cycles or more.

In the test for adhesion of the bonding layer to the current collectoras the evaluation test (4), as is apparent from Table 4, AA-g-PVdFsynthesized by using PVdF having an absorption dose of 1 kGy to 120 kGyis suited for used as the binder for bonding layer of the battery as aresult of analysis of the bonding layer of the battery after repeatingcharge-discharge cycles. To obtain stable adhesion characteristics, PVdFhaving a γ-ray absorption dose of 20 to 100 kGy is preferably used as araw material.

EXAMPLE 17

First, as the third binder, 2 g of AA-g-PVdF obtained by graftpolymerization of 17% by weight of acrylic acid and PVdF was prepared. 2g of AA-g-PVdF was dissolved in 98 g of DMA as a solvent using ahomogenizer to obtain a polymer solution. As a conductive material, 8 gof powdered graphite having a specific surface area of 150 m²/g wasprepared and the powdered graphite was dispersed in 80 g of DMA toprepare a dispersion. The resulting dispersion was added to the abovepolymer solution to prepare a slurry for a bonding layer.

An Al foil having a thickness of 20 μm and a width of 250 mm wasprepared as a positive electrode current collector and the slurry for abonding layer was applied on the Al foil by a doctor blade method andwas then dried to obtain an Al foil with a bonding layer having a drythickness of 5 μm. A Cu foil having a thickness of 10 μm and a width of250 mm was prepared as a negative electrode current collector and theslurry for a bonding layer was applied on the Cu foil by a doctor blademethod and was then dried to obtain a Cu foil with a bonding layerhaving a dry thickness of 5 μm.

The above-mentioned respective components shown in Table 1 were mixed ina ball mill for 2 hours to prepare a coating slurry for a positiveelectrode active material layer, a coating slurry for a negativeelectrode active material layer and coating slurry for an electrolytelayer.

The resulting coating slurry for a positive electrode active materiallayer was applied on the surface of the Al foil having the bonding layerby a doctor blade method so that the dry thickness of the positiveelectrode active material layer was 80 μm, was dried, and was thenrolled to form a positive electrode. Similarly, the coating slurry for anegative electrode active material layer was applied on the surface ofthe Cu foil having the bonding layer by a doctor blade method so thatthe dry thickness of the positive electrode active material layer was 80μm, was dried, and was then rolled to form a negative electrode. Theresulting coating slurry for an electrolyte layer was applied on arelease paper having a thickness of 25 μm and a width of 250 mm by adoctor blade method so that the dry thickness of the electrolyte layerwas 50 μm, and it was then dried to form an electrolyte layer. Thepositive electrode, the electrolyte layer and the negative electrodewere laminated with each other in this order and the resulting laminatewas subjected to thermal compression bonding to form a sheet-likeelectrode body.

A positive electrode lead and a negative electrode lead, each made ofNi, were respectively connected to a positive electrode currentcollector and a negative electrode current collector, and then theresulting electrode bodies were housed by a laminate packaging materialformed into a bag having an opening portion and the opening portion wassealed by thermal compression bonding under reduced pressure to obtain asheet-like battery.

EXAMPLE 18

In the same manner as in Example 17, except that the modified polymerused in the bonding layer was replaced by a modified polymer(methacrylic acid grafting polyvinylidene fluoride, MA-g-PVdF)represented by the above chemical scheme (2) obtained by graftpolymerization of 17% by weight of methacrylic acid and polyvinylidenefluoride, a battery was produced.

EXAMPLE 19

In the same manner as in Example 17, except that the content of anacidic polymer dispersant was controlled to 10.7% by weight based on theweight of the solid material by adding 1.2 g of a dispersant when aconductive material is dispersed in a solvent in the preparation of aslurry for a bonding layer, a battery was produced.

EXAMPLE 20

In the same manner as in Example 18, except that the content of anacidic polymer dispersant was controlled to 10.7% by weight based on theweight of the solid material by adding 1.2 g of a dispersant when aconductive material is dispersed in a solvent in the preparation of aslurry for a bonding layer, a battery was produced.

EXAMPLE 21

In the same manner as in Example 17, except that the dry thickness ofthe bonding layer formed on the positive electrode current collector andthe dry thickness of the bonding layer formed on the negative electrodecurrent collector were respectively controlled to 0.5 μm, a battery wasproduced.

EXAMPLE 22

In the same manner as in Example 17, except that the dry thickness ofthe bonding layer formed on the positive electrode current collector andthe dry thickness of the bonding layer formed on the negative electrodecurrent collector were respectively controlled to 1 μm, a battery wasproduced.

EXAMPLE 23

In the same manner as in Example 17, except that the dry thickness ofthe bonding layer formed on the positive electrode current collector andthe dry thickness of the bonding layer formed on the negative electrodecurrent collector were respectively controlled to 10 μm, a battery wasproduced.

EXAMPLE 24

In the same manner as in Example 17, except that the dry thickness ofthe bonding layer formed on the positive electrode current collector andthe dry thickness of the bonding layer formed on the negative electrodecurrent collector were respectively controlled to 15 μm, a battery wasproduced.

EXAMPLE 25

In the same manner as in Example 17, except that a weight ratio of thebinder to the conductive material was controlled to 50/50 by using 2 gof powdered graphite as the conductive material of the coating slurryfor a bonding layer, a battery was produced.

EXAMPLE 26

In the same manner as in Example 17, except that a weight ratio of thebinder to the conductive material was controlled to 33/67 by using 4 gof powdered graphite as the conductive material of the coating slurryfor a bonding layer, a battery was produced.

EXAMPLE 27

In the same manner as in Example 17, except that a weight ratio of thebinder to the conductive material was controlled to 14/86 by using 12 gof powdered graphite as the conductive material of the coating slurryfor a bonding layer, a battery was produced.

EXAMPLE 28

In the same manner as in Example 17, except that a weight ratio of thebinder to the conductive material was controlled to 13/87 by using 14 gof powdered graphite as the conductive material of the coating slurryfor a bonding layer, a battery was produced.

COMPARATIVE EXAMPLE 9

In the same manner as in Example 17, except that the bonding layer wasnot provided on the positive electrode current collector and thenegative electrode current collector, a battery was produced.

COMPARATIVE EXAMPLE 10

In the same manner as in Example 17, except that the binder AA-g-PVdF ofthe coating slurry for a bonding layer was replaced by a butyl rubberand the solvent DMA was replaced by toluene, a battery was produced.

COMPARATIVE EXAMPLE 11

In the same manner as in Example 17, except that the binder AA-g-PVdF ofthe coating slurry for a bonding layer was replaced by an acrylateester-methacrylate ester copolymer and the solvent DMA was replaced bywater, a battery was produced.

COMPARATIVE EXAMPLE 12

In the same manner as in Example 17, except that the binder AA-g-PVdF ofthe coating slurry for a bonding layer was replaced by a polyurethaneresin and the solvent DMA was replaced by a mixed solvent of 108 g ofmethyl ethyl ketone and 72 g of methyl isobutyl ketone, a battery wasproduced.

COMPARATIVE EXAMPLE 13

In the same manner as in Example 17, except that the binder AA-g-PVdF ofthe coating slurry for a bonding layer was replaced by an epoxy resinand the solvent DMA was replaced by a mixed solvent of 108 g of methylethyl ketone and 72 g of methyl isobutyl ketone, a battery was produced.

COMPARATIVE EXAMPLE 14

In the same manner as in Example 17, except that the content of anacidic polymer dispersant was controlled to 26% by weight by adding 3.5g of a dispersant when a conductive material was dispersed in a solventin the preparation of a slurry for a bonding layer, a battery wasproduced.

COMPARATIVE EXAMPLE 15

In the same manner as in Example 17, except that the dry thickness ofthe bonding layer formed on the positive electrode current collector andthe dry thickness of the bonding layer formed on the negative electrodecurrent collector were respectively controlled to 40 μm, a battery wasproduced.

COMPARATIVE EXAMPLE 16

In the same manner as in Example 17, except that a weight ratio of thebinder to the conductive material was controlled to 9/91 by using 20 gof powdered graphite as the conductive material of the coating slurryfor a bonding layer, a battery was produced.

Comparative Evaluation

With respect to the batteries obtained in Examples 17 to 28 andComparative Examples 9 to 16, the following evaluation tests wereconducted.

(5) Test for Resistance of Bonding Layer to Electrolytic Solution

The negative electrode current collectors having a bonding layerobtained in Examples 17 to 28 and Comparative Examples 9 to 16 weredipped in an electrolytic solution comprising 20 parts by weight ofpropylene carbonate, 40 parts by weight of ethylene carbonate and 40parts by weight of diethyl carbonate for one week, and then an increasein weight of the negative electrode current collectors having a bondinglayer was measured, and it was determined whether or not the modifiedpolymer as the third binder of the bonding layer is swollen with theelectrolytic solution. Also, it was determined whether or not thebonding layer was peeled off by rubbing the Cu foils as the negativeelectrode current collectors with the fingers.

(6) Test for Current Collector Protection Performances of Bonding Layerto Hydrogen Fluoride (HF)

On the surface of the negative electrode current collectors having abonding layer obtained in Examples 17 to 28 and Comparative Examples 9to 16, 2 ml of an aqueous hydrofluoric acid solution having aconcentration of 5 ppm was added dropwise, and after standing for 24hours, the state of the current collectors was determined.

(7) Test for Adhesion of Bonding Layer to Active Material Layer

On the surface of each of the positive electrode current collectors andthe negative electrode current collectors of positive and negativeelectrodes obtained in Examples 17 to 28 and Comparative Examples 9 to16, an adhesive tape was applied and a rubber roller was pressed againstthe surface. The positive electrode current collectors and negativeelectrode current collectors with the adhesive tape applied thereon werecut into pieces having a width of 10 mm, and then current collectorshaving a width of 10 mm was vertically pulled upward and a forcerequired to peel off the active material layer was measured. Also themanner of peeling was visually confirmed.

(8) Test for Cycle Capacity Maintaining Characteristics

After the sheet-like batteries obtained in Examples 17 to 28 andComparative Examples 9 to 16 were subjected to the test forcharge-discharge cycles, a charge-discharge cycle comprising a chargestep of charging the resulting sheet-like battery under the conditionsof a maximum charge voltage of 4.2 V and a charge current of 0.5 A for2.5 hours and a discharge step of discharging at a constant current of0.5 A until the discharge voltage was reduced to 2.75 V (minimumdischarge voltage) was repeated. Then, a charge-discharge capacity ofeach cycle was measured and the number of cycles required to reduce to80% of an initial discharge capacity was measured.

The results in the evaluation tests (5) to (8) are shown in Table 5.Symbols in the column of the evaluation test (5) shown in Table 5 havethe following meanings.

-   ⊚: excellent adhesion; no peeling-   ◯: good adhesion; no peeling-   Δ: partial peeling-   ×: complete peeling

Symbols in the column of the evaluation test (6) shown in Table 5 havethe following meanings.

-   ⊚: excellent corrosion resistance; no corrosion-   ◯: good corrosion resistance; no corrosion-   Δ: partial corrosion-   ×: corrosion

TABLE 5 Evaluation test (5) Dipping in electrolytic EvaluationEvaluation Evaluation solution test (6) test (7) test (8) Increase PeelProtection Adhesion force 80% Capacity cycle [% by weight] resistanceability to HF {N} number [times] Example 17 0.8 ⊚ ⊚ 12.1 635 Example 181.1 ⊚ ⊚ 11.8 565 Example 19 0.9 ⊚ ⊚ 12.3 604 Example 20 1.0 ⊚ ⊚ 11.8 539Example 21 0.5 ⊚ ◯ 12.2 346 Example 22 0.7 ⊚ ◯ 9.0 545 Example 23 1.9 ⊚⊚ 10.8 589 Example 24 2.5 ◯ ⊚ 10.6 444 Example 25 0.4 ⊚ ⊚ 12.0 329Example 26 0.6 ⊚ ⊚ 11.0 566 Example 27 1.2 ◯ ◯ 10.0 468 Example 28 1.8 ◯◯ 9.3 375

Evaluation test (5) Dipping in electrolytic Evaluation EvaluationEvaluation solution test (6) test (7) test (8) Increase ProtectionAdhesion force 80% Capacity cycle [% by weight] Peel resistance abilityto HF {N} number [times] Comparative — — X 0.5 15 Example 9 Comparative83.2 X Δ 2.1 135 Example 10 Comparative 34.8 Δ X 3.6 194 Example 11Comparative 51.1 Δ Δ 3.7 203 Example 12 Comparative 25.7 Δ Δ 2.6 178Example 13 Comparative 0.9 Δ ◯ 5.6 280 Example 14 Comparative 6.0 Δ ⊚ 14213 Example 15 Comparative 2.9 Δ ◯ 4.8 187 Example 16(5) Test for Resistance of Bonding Layer to Electrolytic Solution

In Comparative Examples 10 to 13, the increase in weight of the negativeelectrode current collector having a bonding layer is large and thebinder, which forms the bonding layer, is swollen with the electrolyticsolution. On the other hand, in Examples 17 to 28, regardless ofimmersion in the electrolytic solution for one week, the increase inweight of the negative electrode current collectors having a bondinglayer is very small and the negative electrode current collectors areexcellent in resistance to the electrolytic solution.

(6) Test for Current Collector Protection Performances of Bonding Layerto Hydrogen Fluoride (HF)

The current collectors of Comparative Examples 9 and 11 were corroded byHF. Also the current collectors of Comparative Example 10, 12, and 13were partially corroded. On the other hand, the current collectors ofExamples 17 to 28 are not corroded by HF and it was found that thebonding layer exerts a protective effect.

(7) Test for Adhesion of Bonding Layer to Active Material Layer

As compared with the adhesion of Comparative Examples 9 to 16, a forcerequired to peel off the active material layer is large in Examples 17to 28. In Comparative Examples 9 to 16, the peeled portion and thenon-peeled portion simultaneously existed at the surface. In contrast,the active material layer was uniformly peeled off over the entiresurface in Examples 17 to 28.

(8) Test for Cycle Capacity Maintaining Characteristics

As compared with 80% capacity cycle number of Comparative Examples 9 to16, the batteries of Examples 17 to 28 exhibit high cycle number, and itwas found that these batteries were excellent in cycle maintainingcharacteristics due to charge and discharge.

EXAMPLE 29

First, 2 g of a polymer material (acrylic acid grafting polyvinylidenefluoride, hereinafter referred to as AA-g-PVdF) obtained by graftpolymerization of 17% by weight of acrylic acid and polyvinylidenefluoride was prepared as a main component of first and second binders. 2g of the resulting AA-g-PVdF was dissolved in 98 g of dimethylacetamide(hereinafter referred to as DMA) as a solvent using a homogenizer toobtain first and second binder material solutions. 8 g of aluminumhaving a particle size of 1 μm was prepared as a first conductivematerial and 8 g of copper having a particle size of 1 μm was preparedas a second conductive material, and then these metal powders weredispersed in 80 g of DMA to prepare first and second dispersions. Theresulting first and second dispersions were added to the above bindermaterial solution to prepare slurries for first and second bondinglayers.

An Al foil having a thickness of 20 μm and a width of 250 mm wasprepared as a positive electrode current collector, and then the slurryfor a first bonding layer was applied on the Al foil and was dried toobtain an Al foil with a first bonding layer having a dry thickness of 5μm. A copper foil having a thickness of 10 μm and a width of 250 mm wasprepared as a negative electrode current collector, and then the slurryfor a second bonding layer was applied on the Cu foil and was dried toobtain a copper foil with a second bonding layer having a dry thicknessof 5 μm.

The above-mentioned respective components shown in Table 6 below weremixed in a ball mill for 2 hours to prepare a coating slurry for apositive electrode active material layer, a coating slurry for anegative electrode active material layer, and a coating slurry for anelectrolyte layer.

TABLE 6 Positive electrode Parts Negative electrode Parts Parts activematerial by active material by by component weight component weightPolymer electrolyte component weight LiCoO₂ 90 Powdered graphite 90Vinylidene 17 fluoride-hexafluoropropylene copolymer Powdered graphite 6Polyvinylidene 10 Propylene carbonate 15 fluoride Polyvinylidene 4N-methyl-pyrrolidone 50 Ethylene carbonate 30 fluorideN-methyl-pyrrolidone 45 Diethyl carbonate 30 LiPF₆ 8 Acetone 80

The resulting coating slurry for a positive electrode active materiallayer was applied on the surface of the Al foil having the bonding layerso that the dry thickness of the positive electrode active materiallayer was 80 μm, was dried, and was then rolled to form a positiveelectrode. The resulting coating slurry for a negative electrode activematerial layer was applied on the surface of the Cu foil having thebonding layer so that the dry thickness of the positive electrode activematerial layer was 80 μm, was dried, and was then rolled to form anegative electrode. The resulting coating slurry for an electrolytelayer was applied on a release paper having a thickness of 25 μm and awidth of 250 mm so that the dry thickness of the electrolyte layer was50 μm, and it was then dried to form an electrolyte layer. The positiveelectrode, the electrolyte layer, and the negative electrode werelaminated with each other, in this order, and the resulting laminate wassubjected to thermal compression bonding to form a sheet-like electrodebody.

A positive electrode lead and a negative electrode lead, each made ofNi, were respectively connected to a positive electrode currentcollector and a negative electrode current collector, and then theresulting electrode bodies were housed by a laminate packaging materialformed into a bag having an opening portion, and the opening portion wassealed by thermal compression bonding under reduced pressure to obtain asheet-like battery.

EXAMPLE 30

In the same manner as in Example 29, except that the particle size ofaluminum as the first conductive material and the particle size ofcopper as the second conductive material were changed to 0.1 μm, alithium ion polymer secondary battery was produced.

EXAMPLE 31

In the same manner as in Example 29, except that the particle size ofaluminum as the first conductive material and the particle size ofcopper as the second conductive material were changed to 20 μm, alithium ion polymer secondary battery was produced.

EXAMPLE 32

In the same manner as in Example 29, except that the weight ratio of thefirst binder to the first conductive material (first binder/firstconductive material) and the weight ratio of the second binder to thesecond conductive material (second binder/second conductive material)were respectively changed to 13/87, a lithium ion polymer secondarybattery was produced.

EXAMPLE 33

In the same manner as in Example 29, except that the weight ratio of thefirst binder to the first conductive material (first binder/firstconductive material) and the weight ratio of the second binder to thesecond conductive material (second binder/second conductive material)were respectively changed to 75/25, a lithium ion polymer secondarybattery was produced.

EXAMPLE 34

In the same manner as in Example 29, except that platinum was used asthe first conductive material, a lithium ion polymer secondary batterywas produced.

EXAMPLE 35

In the same manner as in Example 29, except that nickel was used as thesecond conductive material, a lithium ion polymer secondary battery wasproduced.

EXAMPLE 36

In the same manner as in Example 29, except that the acidic polymerdispersant was added to the first and second bonding layers, a lithiumion polymer secondary battery was produced.

EXAMPLE 37

In the same manner as in Example 29, except that the neutral polymerdispersant was added to the first and second bonding layers, a lithiumion polymer secondary battery was produced.

COMPARATIVE EXAMPLE 17

In the same manner as in Example 29, except that a powdered carbonmaterial having a specific surface area of 150 cm²/g was used as thefirst and second conductive materials, a lithium ion polymer secondarybattery was produced.

COMPARATIVE EXAMPLE 18

In the same manner as in Example 29, except that the particle size ofaluminum as the first conductive material and the particle size ofcopper as the second conductive material were changed to 0.05 μm, alithium ion polymer secondary battery was produced.

COMPARATIVE EXAMPLE 19

In the same manner as in Example 29, except that the particle size ofaluminum as the first conductive material and the particle size ofcopper as the second conductive material were changed to 25 μm, alithium ion polymer secondary battery was produced.

COMPARATIVE EXAMPLE 20

In the same manner as in Example 29, except that the weight ratio of thefirst binder to the first conductive material (first binder/firstconductive material) and the weight ratio of the second binder to thesecond conductive material (second binder/second conductive material)were respectively changed to 10/90, a lithium ion polymer secondarybattery was produced.

COMPARATIVE EXAMPLE 21

In the same manner as in Example 29, except that the weight ratio of thefirst binder to the first conductive material (first binder/firstconductive material) and the weight ratio of the second binder to thesecond conductive material (second binder/second conductive material)were respectively changed to 80/20, a lithium ion polymer secondarybattery was produced.

Comparative Evaluation

With respect to the batteries obtained in Examples 29 to 37 andComparative Examples 17 to 21, the following evaluation tests wereconducted.

(1) Tear Off Test

This test was conducted by using a tensile testing machine (TensilonUCT-500, manufactured by Orientec Co., Ltd.) after cutting a sheet-likeelectrode body into pieces having a width of 50 mm. After the electrodebody having a width of 50 mm was fixed by a pair of chucks having adistance of 100 mm, one chuck was pulled at a pulling rate of 300 mm/minand the load required to break the electrode body was measured. Theresulting value was taken as the adhesion force.

(2) Test for Output Characteristics

After these batteries were charged until a voltage between terminals ofbatteries was 4.0 [V] at a current value represented by ⅕ C [mA] in acase in which each discharge capacity of the batteries is expressed as 1C [mAh], they were charged while being maintained at 4.0 [V] for 5 hoursin total and were allowed to stand for one hour. Then, the dischargecapacity was measured after discharging until the voltage was 2.7 [V] ata current value of 3 C [mA]. A percentage of a discharge capacityobtained when discharged at a current value expressed by 3 C [mA] to adischarge capacity obtained when discharged at a current value expressedby ⅕ C [mA] was determined.

(3) Test for Cycle Capacity Maintaining Characteristics

Cycle lifetime of these batteries was determined by the number ofcharge-discharge cycles until the discharge capacity is reduced to 80%of an initial discharge capacity C_(MAX).

The results of the evaluation tests (1) to (3) are respectively shown inTable 7.

TABLE 7 Adhesion Output Cycle force characteristics characteristics [N][%] [times] Example 29 59.2 95.8 851 Example 30 47 97.4 699 Example 3151.2 94.6 893 Example 32 67 98.7 658 Example 33 46.2 92.4 994 Example 3455.8 97.7 814 Example 35 58 96.7 772 Example 36 52.8 97.9 781 Example 3757 96.3 732 Comparative 36.2 90.8 635 Example 17 Comparative 40.8 90.2534 Example 18 Comparative 40 87.5 473 Example 19 Comparative 25.6 88.5139 Example 20 Comparative 45.8 67.5 758 Example 21

As is apparent from the results of the tear off test shown in Table 7,the battery of Comparative Example 17, wherein no metal is used in thefirst and second bonding layers, has low adhesion force and is likely tocause peeling. In the batteries of Comparative Examples 18 and 19wherein the particle size of metal contained in the first and secondbonding layers is not within the scope of the present invention, sinceparticles having a small particle size agglomerated (Comparative Example18) and particles have a large particle size and are coarse, and thus,conductive materials are not sufficiently in contact (ComparativeExample 19), the results of the respective evaluation tests exhibitedlow numerical values. The battery of Comparative Example 20 was inferiorin adhesion and cycle characteristics, while the battery of ComparativeExample 21 was inferior in output characteristics. On the other hand,the batteries of Examples 29 to 37 are excellent in adhesion, outputcharacteristics, and cycle characteristics, as compared with the batteryof the prior art.

EXAMPLE 38

2 g of a particulate acrylic acid grafting polyvinylidene fluoridehaving a mean particle size of 200 μm (hereinafter referred to asAA-g-PVdF) as a binder and 98 g of dimethylacetamide (hereinafterreferred to as DMA) was prepared as a solvent, and then the two weremixed. The resulting mixed solution was heated to 85° C. andcontinuously stirred using a homogenizer. The solution was collectedwhile stirring and applied on a transparent glass substrate so that theresulting liquid film had a thickness of about 200 μm, and then a meanparticle size of polymer binder particles (undissolved particles) andthe number of polymer binder particles (undissolved particles) having aparticle size of 1 μm or more were determined by using an opticalmicroscope.

The particle size of the particles was measured by definition of aprojected equivalent circle diameter and its measuring method. Theparticle size was measured as a diameter of a circle equivalent to aprojected area of the particles by the following procedure. That is, theparticles arranged in a plane were observed from directly above using ameans such as an optical microscope, electron microscope or close-upphotography, and the particle size was determined by the resultingprojected drawing of the particles. Then, a mean particle size D wasdetermined by weighting according to the following equation:D=[Σnd ³ /Σn] ^(1/3)where d denotes a mean particle size obtained by the above method and ndenotes the number of particles.

When the mean particle size of the polymer binder particles became 30±10μm and the number of the polymer binder particles having a particle sizeof 1 μm or more became 20±10/cm², stirring was stopped. A solutioncontaining the resulting polymer binder particles was taken as a polymersolution of Example 38.

EXAMPLE 39

1.5 g of particulate AA-g-PVdF having a mean particle size of 200 μm wasprepared as a binder and then mixed with 98 g of DMA. The resultingmixed solution was heated to 85° C. and continuously stirred by using ahomogenizer. The resulting AA-g-PVdF was completely dissolved and 0.5 gof particulate AA-g-PVdF having a mean particle size of 100 μm werefurther added, followed by stirring for an additional 5 minutes using ahomogenizer to obtain a polymer binder particle-containing polymersolution. The resulting solution was collected and applied on atransparent glass substrate so that the resulting liquid film had athickness of about 200 μm, and then the mean particle size of polymerbinder particles (undissolved particles) and the number of polymerbinder particles (undissolved particles) having a particle size of 1 μmor more were determined by an optical microscope. As a result, the meanparticle size of the polymer binder particles was 20±10 μm and thenumber of the polymer binder particles having a particle size of 1 μm ormore was 20±10/cm². The resulting polymer binder particle-containingsolution was taken as a polymer solution of Example 39.

EXAMPLE 40

In the same manner as in Example 38, 2 g of AA-g-PVdF having a meanparticle size of 200 μm and 98 g of DMA were prepared and the two werecontinuously stirred under the same conditions as in Example 38. Theresulting solution was often collected, and then the mean particle sizeof polymer binder particles (undissolved particles) and the number ofpolymer binder particles (undissolved particles) having a particle sizeof 1 μm or more were determined by an optical microscope under the sameconditions as in Example 38. When the mean particle size of the polymerbinder particles became 10±5 μm and the number of the polymer binderparticles having a particle size of 1 μm or more became 10±5/cm²,stirring was stopped. The resulting polymer binder particles-containingsolution was taken as a polymer solution of Example 40.

EXAMPLE 41

2 g of a particulate methacrylic acid grafting polyvinylidene fluoridehaving a mean particle size of 200 μm was prepared as a binder and 98 gof DMA were prepared, and then the two were continuously stirred underthe same conditions as in Example 38. The resulting solution was oftencollected, and then the mean particle size of polymer binder particles(undissolved particles) and the number of polymer binder particles(undissolved particles) having a particle size of 1 μm or more weredetermined by an optical microscope under the same conditions as inExample 38. When the mean particle size of the polymer binder particlesbecame 30±10 μm and the number of the polymer binder particles having aparticle size of 1 μm or more became 20±10/cm², stirring was stopped.The resulting polymer binder particle-containing solution was taken as apolymer solution of Example 41.

COMPARATIVE EXAMPLE 22

In the same manner as in Example 38, 2 g of AA-g-PVdF having a meanparticle size of 200 μm was prepared as a binder and then mixed with 98g of DMA. The resulting mixed solution was heated to 85° C. and wascontinuously stirred using a homogenizer until AA-g-PVdF is completelydissolved. The resulting solution was collected and applied on atransparent glass substrate so that the resulting liquid film had athickness of about 200 μm, and then it was confirmed by an opticalmicroscope that polymer binder particles (undissolved particles) havinga particle size of 1 μm or more were not present. The resulting polymerbinder particle-containing solution was taken as a polymer solution ofComparative Example 22.

COMPARATIVE EXAMPLE 23

In the same manner as in Example 39, 1.5 g of AA-g-PVdF having a meanparticle size of 200 μm was prepared as a binder and was then mixed with98 g of DMA. The resulting mixed solution was heated to 85° C. and wascontinuously stirred using a homogenizer. AA-g-PVdF was completelydissolved and 0.5 g of the particulate AA-g-PVdF having a mean particlesize of 200 μm was further added, followed by stirring for an additionalone minute using a homogenizer to obtain a polymer solution. Theresulting solution was collected and applied on a transparent glasssubstrate so that the resulting liquid film had a thickness of about 200μm, and then the mean particle size of polymer binder particles(undissolved particles) and the number of polymer binder particles(undissolved particles) having a particle size of 1 μm or more weredetermined by using an optical microscope. As a result, the meanparticle size of the polymer binder particles was 120±10 μm and thenumber of the polymer binder particles having a particle size of 1 μm ormore was 80±10/cm². The resulting polymer binder particles-containingsolution was taken as a polymer solution of Example 23.

COMPARATIVE EXAMPLE 24

In the same manner as in Example 39, 1 g of AA-g-PVdF having a meanparticle size of 200 μm was prepared as a binder and was then mixed with98 g of DMA. The resulting mixed solution was heated to 85° C. and wascontinuously stirred using a homogenizer. AA-g-PVdF was completelydissolved and 1 g of the particulate AA-g-PVdF having a mean particlesize of 100 μm was further added, followed by stirring for an additional2 minutes using a homogenizer to obtain a polymer solution. Theresulting solution was collected and applied on a transparent glasssubstrate so that the resulting liquid film had a thickness of about 200μm, and then the mean particle size of polymer binder particles(undissolved particles) and the number of polymer binder particles(undissolved particles) having a particle size of 1 μm or more weredetermined by using an optical microscope. As a result, the meanparticle size of the polymer binder particles was 60±10 μm and thenumber of the polymer binder particles having a particle size of 1 μm ormore was 150±10/cm². The resulting polymer binder particles-containingsolution was taken as a polymer solution of Example 24.

COMPARATIVE EXAMPLE 25

In the same manner as in Example 41, 2 g of a particulate methacrylicacid grafting polyvinylidene fluoride having a mean particle size of 200μm, as a binder, was mixed with 98 g of DMA. The resulting mixedsolution was heated to 85° C. and was stirred using a homogenizer untilpolytetrafluoroethylene is completely dissolved. The resulting solutionwas collected and was applied on a transparent glass substrate so thatthe resulting liquid film had a thickness of about 200 μm, and then itwas confirmed by using an optical microscope that polymer binderparticles (undissolved particles) having a particle size of 1 μm or morewere not present. The resulting polymer binder particle-containingsolution was taken as a polymer solution of Comparative Example 25.

Comparative Test and Evaluation

(1) Test for Adhesion with Copper Foil and Aluminum Foil

First, each of the polymer solutions obtained in Examples 38 to 41 andComparative Examples 22 to 25 was uniformly applied on a Cu foil havinga width of 30 mm, a length of 200 mm and a thickness of 14 μm, thesurface of which was degreased, and then an Al foil having a width of 10mm, a length of 100 mm, and a thickness of 20 μm, the surface of whichwas degreased, was applied thereon to produce a peel specimen foradhesion testing. The resulting specimen was dried in atmospheric air at80° C. for 5 days using a dryer. Then, the binding of the dried specimento the Cu foil and the Al foil was evaluated by a machine for peelingtesting. In the peeling test, the Cu foil side of the specimen was fixedto a testing stand and the Al foil bonded to the Cu foil was verticallypulled upward at a rate of 100 mm/min, and then a force required to peeloff the Al foil from the Cu foil (peel strength) was measured. Thepolymer solution of Example 38 was applied on a copper foil and thendried. An electron micrograph of the binder particles is shown in FIG.10.

(2) Test for Cycle Capacity Maintaining Characteristics of Battery WhenUsed in Bonding Layer of Battery

First, to 100 g of each of the polymer solutions obtained in Examples 38to 41 and Comparative Examples 22 to 25, 8 g of powdered graphite havinga specific surface area of 150 m²/g and 80 g of DMA were added toprepare a slurry for a bonding layer. After preparing an Al foil havinga thickness of 20 μm and a width of 250 μm as a positive electrodecurrent collector, the resulting slurry for a bonding layer was appliedon the Al foil by a doctor blade method and was then dried. Thethickness of the bonding layer after drying was controlled within arange of 20±1 μm. After preparing a Cu foil having a thickness of 14 μmand a width of 250 μm as a negative electrode current collector, theresulting slurry for a bonding layer was applied on the Cu foil by adoctor blade method and was then dried. The thickness of the bondinglayer after drying was controlled within a range of 20±1 μm. Therespective components shown in Table 8 below were mixed in a ball millfor 2 hours to prepare a coating slurry for a positive electrode activematerial layer, a coating slurry for a negative electrode activematerial layer and coating slurry for an electrolyte layer.

TABLE 8 Coating slurry component Parts by weight Positive electrodeLiCoO₂ 90 active material Powdered graphite 6 layer PVdF 4 N-methylpyrrolidone 45 Negative electrode Powdered graphite 90 active materialPVdF 10 layer N-methyl pyrrolidone 50 Electrolyte layer Vinylidene 17fluoride-hexafluoropropylene copolymer Propylene carbonate 15 Ethylenecarbonate 30 Diethyl carbonate 30 LiPF₆ 8 Acetone 80

The resulting coating slurry for a positive electrode active materiallayer was applied on the surface of the Al foil having the bonding layerby a doctor blade method so that the dry thickness of the positiveelectrode active material layer was 80 μm, was dried, and was thenrolled to form a positive electrode. Similarly, the coating slurry for anegative electrode active material layer was applied on the surface ofthe Cu foil having the bonding layer by a doctor blade method so thatthe dry thickness of the positive electrode active material layer was 80μm, was dried, and was then rolled to form a negative electrode.Furthermore, the coating slurry for an electrolyte layer wasrespectively applied on the positive electrode and the negativeelectrode by a doctor blade method so that each dry thickness was 50 μm,and then the positive and negative electrodes, each having anelectrolyte layer, were laminated with each other by thermal compressionbonding to yield a sheet-like electrode bodies. In the resultingsheet-like electrode bodies, a positive electrode lead and a negativeelectrode lead, each made of Ni, were respectively connected to apositive electrode current collector and a negative electrode currentcollector, and then the sheet-like electrode bodies were housed by alaminate packaging material formed into a bag having an opening portion,and the opening portion was sealed by thermal compression bonding underreduced pressure to obtain a sheet-like battery.

Then, a charge-discharge cycle comprising a charge step of charging theresulting sheet-like battery under the conditions of a maximum chargevoltage of 4 V and a charge current of 0.5 A for 2.5 hours, and adischarge step of discharging at a constant current of 0.5 A until thedischarge voltage was reduced to a minimum discharge voltage of 2.75 V,was repeated, and then a charge-discharge capacity of each cycle wasmeasured, and the number of cycles required to reduce to 80% of aninitial discharge capacity was measured.

(3) Test for Adhesion of Bonding Layer to Current Collector When Used inBonding Layer of Battery

First, the same sheet-like batteries as those for the above-mentionedevaluation test (2) were prepared by using the polymer solutionsobtained in Examples 38 to 41 and Comparative Examples 22 to 25. Underthe conditions of 70° C., 100 charge-discharge cycles of the resultingbatteries were conducted under the same conditions as those in theevaluation test (2). After the completion of 100 charge-dischargecycles, the housing package of the sheet-like battery was removed andthe positive electrode and the negative electrode of the battery weredetached and separated. It was then determined whether or not thebonding layer would peeled off from the current collector when pullingthe bonding layers of the separated positive and negative electrodesusing forceps.

The evaluation results in the evaluation tests (1) to (4) are shown inTable 9. Symbols in the column of the evaluation test (3) in Table 9have the following meanings.

-   ⊚: excellent adhesion of bonding layer to current collector; not    peeled off-   ◯: bonding layer was partially peeled off from current collector-   ×: bonding layer was completely peeled off from current collector

TABLE 9 Evaluation Evaluation test (4) test (3) Adhesion of bondingEvaluation test (2) 80% Capacity layer to current Peel strength cyclecollector after 100 cycles [N/cm] of Cu and Al number [times] ofPositive electrode Negative electrode bonded battery current collectorcurrent collector Example 38 20.58 680 ⊚ ⊚ Example 39 19.60 675 ⊚ ⊚Example 40 18.62 657 ⊚ ⊚ Example 41 19.70 648 ⊚ ◯ Comparative 15.68 523⊚ ⊚ Example 22 Comparative 13.72 506 ⊚ ⊚ Example 23 Comparative 13.72459 ⊚ ◯ Example 24 Comparative 15.29 495 ⊚ ◯ Example 25

As is apparent from Table 9, in the test for adhesion with the Cu foiland the Al foil as the evaluation test (1), any Al foil bonded to the Cufoil using the polymer solutions of Examples 38 to 4 exhibited a peelstrength from the Cu foil of 18.6 N/cm or higher. On the other hand,when using the polymer solutions of Comparative Examples 22 to 25, thepeel strength decreased by 4.41 to 5.88 N/cm as compared with the peelstrength of Examples 38 to 41. This is caused by the presence or absenceof the particulate polymer binder, and it is believed that the adhesionwas improved by the presence of the particulate polymer binder. In thetest for cycle capacity maintaining characteristics as the evaluationtest (2), 80% capacity cycle number of the batteries using the polymerbinder of Examples 38 to 41 is higher than that of the batteries usingthe polymer solutions of Comparative Examples 22 to 25. The reason forthis is believed to be that cycle capacity maintaining characteristicswere improved because the polymer binders of Examples 38 to 41 areexcellent in adhesion and are also excellent in durability to theelectrolytic solution.

In Examples 38 to 40 wherein the number of particles having a meanparticle size of 1 to 100 μm present in the polymer solution is 100/cm²,the mean particle size exceeds 100 μm, or the number of cycles is higherthan that of Comparative Examples 23 and 24 wherein the number ofparticles having a mean particle size of 1 μm more exceed 100/cm².

Furthermore, in the test for adhesion of the bonding layer to thecurrent collector as the evaluation test (3), Examples 38 and 39 as wellas Comparative Examples 22 and 23 exhibited high adhesion. As isapparent from a comparison between Examples 38 to 40 and Example 41,when using methacrylic acid grafting polyvinylidene fluoride as abinder, the adhesion is reduced as compared with the case of usingAA-g-PVdF. However, regarding Example 41 wherein methacrylic acidgrafting polyvinylidene fluoride having comparatively poor adhesioncharacteristics is used as a polymer binder and a portion of the binderis allowed to exist in the bonding layer in the form of particles, theadhesion of the bonding layer to the current collector (evaluation test(3)) after 100 cycles is almost the same as in the case of ComparativeExample 25 using completely dissolved polymer binder. However, regardingthe evaluation test (1) (peel strength of Cu and Al bonded) andevaluation test (2) (80% capacity cycle number of battery) (both of themserve as a measure of adhesion in a broad sense), in Example 41 whereina portion of the binder is allowed to exist in the bonding layer in theform of particles, adhesion in a broad sense is improved andcharacteristics of the battery can be improved as compared withComparative Example 25 using completely dissolved polymer binder

INDUSTRIAL APPLICABILITY

According to the first aspect of the present invention, in a lithium ionpolymer secondary battery, a first bonding layer is interposed betweenthe positive electrode current collector and the positive electrodeactive material layer, and a second bonding layer is interposed betweenthe negative electrode current collector and the negative electrodeactive material layer, the first and second bonding layers contain botha third binder and a conductive material, while the third binder is apolymer compound obtained by modifying either or both of the polymercompounds contained in the first and second binders, or a polymercompound obtained by modifying a polymer compound having any ofrepeating units of the polymer compounds, with a modifying material.Therefore, the modified polymer made of the first or second binder as amain component has high adhesion with the positive electrode activematerial layer or negative electrode active material layer and alsoadhesion with the current collector is remarkably improved bymodification as compared with the case of using the binder of the priorart. As a result, peeling of the active material layer from the currentcollector can be suppressed and electrical conductivity between thecurrent collector and the active material layer is remarkably improved,thus making it possible to improve cycle capacity maintainingcharacteristics. Also, since the electrolytic solution scarcelypenetrates into the modified polymer compound, the bonding layer isstable against an organic solvent in the electrolytic solution and isexcellent in long-term storage stability. Even if strong acid such ashydrofluoric acid is generated in the battery, the modified polymercompound serves as a protective layer and can suppress the corrosion ofthe current collector. According to the first aspect of the presentinvention, such an excellent lithium ion polymer secondary battery isprovided and is useful in industrial fields.

According to the second aspect of the present invention, in a lithiumion polymer secondary battery, a first bonding layer is interposedbetween the positive electrode current collector and the positiveelectrode active material layer, and a second bonding layer isinterposed between the negative electrode current collector and thenegative electrode active material layer, the first and second bondinglayers contain both a third binder and a conductive material, while thethird binder contains a polymer compound obtained by modifying afluorine-containing polymer compound with a modifying material.Therefore, adhesion of the respective bonding layers to the positiveelectrode current collector or negative electrode current collector isremarkably improved as compared with the binder of the prior art. As aresult, peeling of the active material layer from the current collectorcan be suppressed and electrical conductivity between the currentcollector and the active material layer is remarkably improved, thusmaking it possible to improve cycle capacity maintainingcharacteristics. Also since the electrolytic solution scarcelypenetrates into the modified polymer compound, the bonding layer isstable against an organic solvent in the electrolytic solution and isexcellent in long-term storage stability. Even if strong acid such ashydrofluoric acid is generated in the battery, the modified polymercompound serves as a protective layer and can suppress the corrosion ofthe current collector. According to the second aspect of the presentinvention, such an excellent lithium ion polymer secondary battery isprovided and is useful in industrial fields.

According to the third aspect of the present invention, there isprovided a method for synthesizing a third binder suited for use in thefirst and second bonding layers. Therefore, the third aspect of thepresent invention has characteristics which are useful in industrialfields.

A lithium ion polymer secondary battery as the third and fourth aspectsof the present invention is an improvement of a lithium ion polymersecondary battery comprising a positive electrode formed by providing apositive electrode active material layer containing a binder for apositive electrode and a positive electrode active material on thesurface of a positive electrode current collector, a negative electrodeformed by providing a negative electrode active material layercontaining a binder for a negative electrode and a negative electrodeactive material on the surface of a negative electrode currentcollector, and a polymer electrolyte layer interposed between thesurface of the positive electrode active material layer of the positiveelectrode and the negative electrode active material layer of thenegative electrode; wherein a first bonding layer containing a firstbinder and a first conductive material is interposed between thepositive electrode current collector layer and the positive electrodeactive material layer; a second bonding layer containing a second binderand a second conductive material is interposed between the negativeelectrode current collector layer and the negative electrode activematerial layer; a main component of the first binder is a main componentof the binder for a positive electrode and a main component of thesecond binder is a main component of the binder for a negativeelectrode; and metal or partially oxidized metal having a particle sizeof 0.1 to 20 μm is used as the conductive material contained in thefirst and second bonding layers and the metal or partially oxidizedmetal has higher electrical conductivity as compared with the carbonmaterial which has conventionally been used as the conductive material.Therefore, a bonding layer having good electronic conductivity can beformed by adding a small amount of the conductive material. Since adirect current resistance component generated in the battery decreases,a battery having excellent output characteristics (rate characteristics)can be produced. Since excellent electrical conductivity can be obtainedby adding a small amount of the conductive material, a volume ratio ofthe binder material in the bonding layer can be remarkably increased,and thus contact area of the binder material with the active materiallayer and the current collector increases and high adhesion force can beobtained. Furthermore, excellent adhesion and electrical conductivitycan be obtained by controlling the weight ratio of the binder to theconductive material contained in the first and second bonding layerswithin predetermined ranges.

As a result, it is made possible to prevent peeling of the activematerial layer from the current collector due to winding and folding,external impact, and expansion and contraction of the active materialduring charge and discharge, and thus excellent charge-discharge cyclecharacteristics (prolonged lifetime) can be obtained. Therefore, thefourth aspect of the present invention has characteristics which areuseful in industrial fields.

According to the fifth aspect of the present invention, in an electrodefor secondary battery, comprising a current collector and an activematerial layer formed on one or both surfaces of the current collectorvia a bonding layer containing a polymer binder, a portion of thepolymer binder is allowed to exist in the bonding layer in the form ofparticles, and a volume-mean particle size of the particulate polymerbinder is controlled within a range from 1 to 100 μm. Therefore, theparticulate polymer binder that exists in the bonding layer existstogether with the conductive material, that exists in the form ofparticles, in the interface between the current collector and thebonding layer and in the interface between the active material layer andthe bonding layer, thereby improving adhesion with the layers. Theconductive material exists in the portion of interface between thecurrent collector and the bonding layer where the particulate polymerbinder does not exist and in the portion of the interface between theactive material layer and the bonding layer, so that exchange ofelectrons is carried out smoothly in the interface because of thepresence of the conductive material, and the electrical resistance canbe maintained at a low level.

In this case, when a fluororesin is used as a main component of thepolymer binder, an electrode for a secondary battery having highdurability to the electrolytic solution can be obtained. When thepolymer binder is a compound obtained by graft polymerization ofpolyvinylidene fluoride and acrylic acid or methacrylic acid as amonomer, an electrode for secondary battery having excellent adhesionwith the current collector can be obtained. When a surface density ofthe particulate polymer binder in a cross section of the bonding layerparallel to the surface of the bonding layer is from 1 to 100/cm², theparticulate polymer binder is distributed with a proper density in theinterface between the current collector and the bonding layer and theinterface between the active material layer and the bonding layer, andboth adhesion and electrical conductivity in the interface can bemaintained. A secondary battery using this secondary electrode forbattery (sixth aspect of the present invention) has improved cyclecapacity maintaining characteristics.

Therefore, the fifth and sixth aspects of the present invention havecharacteristics which are useful in industrial fields.

1. A lithium ion polymer secondary battery comprising: a positiveelectrode comprising a positive electrode current collector, and apositive electrode active material layer, which contains a first bindercontaining a polymer compound and a positive electrode active material,provided on the surface of the positive electrode current collector, anegative electrode comprising a negative electrode current collector,and a negative electrode active material layer which contains a secondbinder containing a polymer compound that is the same as or differentfrom that of the first binder, and a negative electrode active material,provided on the surface of the negative electrode current collector, andan electrolyte, wherein a first bonding layer is interposed between thepositive electrode current collector and the positive electrode activematerial layer, and a second bonding layer is interposed between thenegative electrode current collector and the negative electrode activematerial layer, the first and second bonding layers contain both a thirdbinder and a conductive material, and the third binder contains apolymer compound obtained by modifying either or both of the polymercompounds contained in the first and second binders or a polymercompound having any of repeating units of the polymer compounds, withmodifying material.
 2. The lithium ion polymer secondary batteryaccording to claim 1, wherein either or both of the first and secondbinders contains a fluorine-containing polymer compound.
 3. The lithiumion polymer secondary battery according to claim 2, wherein thefluorine-containing polymer compound is selected frompolytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidenefluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, andpolyvinyl fluoride.
 4. The lithium ion polymer secondary batteryaccording to claim 1, wherein the modifying material is a compoundselected from ethylene, styrene, butadiene, vinyl chloride, vinylacetate, acrylic acid, methyl acrylate, methyl vinyl ketone, acrylamide,acrylonitrile, vinylidene chloride, methacrylic acid, methylmethacrylate, and isoprene.
 5. The lithium ion polymer secondary batteryaccording to claim 1, wherein the thickness of the first and secondbonding layers is from 0.5 to 30 μm.
 6. The lithium ion polymersecondary battery according to claim 1, which further contains 0.1 to20% by weight of a dispersant in the first and second bonding layers. 7.The lithium ion polymer secondary battery according to claim 1, whereina particle size of the conductive material is from 0.5 to 30 μm, acarbon material having a graphitization degree of 50% or more is used asthe conductive material, and a weight ratio of the third binder to theconductive material contained in the first and second bonding layers,(third binder/conductive material), is from 13/87 to 50/50.
 8. A lithiumion polymer secondary battery comprising: a positive electrodecomprising a positive electrode current collector, and a positiveelectrode active material layer, which contains a first binder and apositive electrode active material, provided on the surface of thepositive electrode current collector, a negative electrode comprising anegative electrode current collector, and a negative electrode activematerial layer which contains a second binder that is the same as ordifferent from that of the first binder, and a negative electrode activematerial, provided on the surface of the negative electrode currentcollector, and an electrolyte, wherein a first bonding layer isinterposed between the positive electrode current collector and thepositive electrode active material layer and a second bonding layer isinterposed between the negative electrode current collector and thenegative electrode active material layer, the first and second bondinglayers contain both the third binder and the conductive material, andthe third binder contains a polymer compound obtained by modifying afluorine-containing polymer compound with a modifying material.
 9. Thelithium ion polymer secondary battery according to claim 8, whereineither or both of the first and second binders contains afluorine-containing polymer compound.
 10. The lithium ion polymersecondary battery according to claim 9, wherein the fluorine-containingpolymer compound contained in either or both of the first and secondbinders is a fluorine-containing polymer compound selected frompolytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidenefluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, andpolyvinyl fluoride.
 11. The lithium ion polymer secondary batteryaccording to claim 8, wherein the fluorine-containing polymer compoundcontained in the third binder is a fluorine-containing polymer compoundselected from polytetrafluoroethylene, polychlorotrifluoroethylene,polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylenecopolymer, and polyvinyl fluoride.
 12. The lithium ion polymer secondarybattery according to claim 8, wherein the modifying material is acompound selected from ethylene, styrene, butadiene, vinyl chloride,vinyl acetate, acrylic acid, methyl acrylate, methyl vinyl ketone,acrylamide, acrylonitrile, vinylidene chloride, methacrylic acid, methylmethacrylate, and isoprene.
 13. The lithium ion polymer secondarybattery according to claim 8, wherein the thickness of the first andsecond bonding layers is from 0.5 to 30 μm.
 14. The lithium ion polymersecondary battery according to claim 8, which further contains 0.1 to20% by weight of a dispersant in the first and second bonding layers.15. The lithium ion polymer secondary battery according to claim 8,wherein a particle size of the conductive material is from 0.5 to 30 μm,a carbon material having a graphitization degree of 50% or more is usedas the conductive material, and a weight ratio of the third binder tothe conductive material contained in the first and second bondinglayers, (third binder/conductive material), is from 13/87 to 50/50. 16.The lithium ion polymer secondary battery according to claim 1, whereinthe first conductive materials contained in the first bonding layer andthe second conductive materials contained in the second bonding layer,contain a metal or partially oxidized metal having a particle size of0.1 to 20 μm, and a weight ratio of the third binder to the firstconductive material contained in the first bonding layer, (thirdbinder/first conductive material), and a weight ratio of the thirdbinder to the conductive material contained in the second bonding layer,(third binder/second conductive material), are from 13/87 to 75/25. 17.The lithium ion polymer secondary battery according to claim 16, whereinthe first and second conductive materials contain mixtures or alloys ofone or more kinds selected from the group consisting of aluminum, steel,iron, nickel, cobalt, silver, gold, platinum, palladium, and partiallyoxidized material of these metals.
 18. The lithium ion polymer secondarybattery according to claim 16, wherein an acidic polymer dispersant, abasic polymer dispersant or a neutral polymer dispersant is furtheradded in the first and second bonding layers.
 19. The lithium ionpolymer secondary battery according to claim 8, wherein the first andsecond conductive materials contain a metal or partially oxidized metalhaving a particle size of 0.1 to 20 μm, and a weight ratio of the thirdbinder to the first conductive material contained in the first bondinglayer, (third binder/first conductive material), and a weight ratio ofthe third binder to the conductive material contained in the secondbonding layer, (third binder/second conductive material), are from 13/87to 75/25.
 20. The lithium ion polymer secondary battery according toclaim 19, wherein the first and second conductive materials containmixtures or alloys of one or more kinds selected from the groupconsisting of aluminum, steel, iron, nickel, cobalt, silver, gold,platinum, palladium, and partially oxidized material of these metals.21. The lithium ion polymer secondary battery according to claim 19,wherein an acidic polymer dispersant, a basic polymer dispersant or aneutral polymer dispersant is further added in the first and secondbonding layers.
 22. An electrode for secondary battery, comprising acurrent collector and an active material layer formed on one or bothsurfaces of the current collector via a bonding layer containing apolymer binder, wherein a portion of the polymer binder exists in thebonding layer in the form of particles, and a volume-mean particle sizeof the particulate polymer binder is from 1 to 100 μm.
 23. The electrodefor secondary battery according to claim 22, wherein a main component ofthe polymer binder is a fluororesin.
 24. The electrode for secondarybattery according to claim 22, wherein the polymer binder is a compoundobtained by graft polymerization of polyvinylidene fluoride and acrylicacid or methacrylic acid.
 25. The electrode for secondary batteryaccording to claim 22, wherein a surface density of the particulatepolymer binder in a cross section of the bonding layer parallel to thesurface of the bonding layer is from 1 to 100/cm².
 26. A secondarybattery comprising the electrode for secondary battery according toclaim
 22. 27. The lithium ion polymer secondary battery according toclaim 1, wherein modification with the modifying material is carried outby irradiating either or both of the polymer compounds contained in thefirst and second binders, or the polymer compound having any ofrepeating units of the polymer compounds, with radiation and mixing themodifying material with the irradiated polymer compound, thereby tocause graft polymerization.
 28. The lithium ion polymer secondarybattery according to claim 8, wherein modification with the modifyingmaterial is carried out by irradiating either or both of the polymercompounds contained in the first and second binders, or the polymercompound having any of repeating units of the polymer compounds, withradiation and mixing the modifying material with the irradiated polymercompound, thereby to cause graft polymerization.
 29. The lithium ionpolymer secondary battery according to claim 1, wherein modificationwith the modifying material is carried out by mixing the modifyingmaterial with either or both of the polymer compounds contained in thefirst and second binders, or the polymer compound having any ofrepeating units of the polymer compounds, and irradiating the mixturewith radiation, thereby to cause graft polymerization.
 30. The lithiumion polymer secondary battery according to claim 8, wherein modificationwith the modifying material is carried out by mixing the modifyingmaterial with either or both of the polymer compounds contained in thefirst and second binders, or the polymer compound having any ofrepeating units of the polymer compounds, and irradiating the mixturewith radiation, thereby to cause graft polymerization.